A/C and heat unit makes "slamming" noise when stops

by Sarah S
(Long Beach, CA)

When my A/C or heat turns off, we hear a fairly loud slamming noise in the house. It sounds like a metal flap slamming shut.

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Aug 08, 2018
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Slamming Sound on Exterior Unit NEW
by: Anonymous

When the unit shuts off, there is a loud banging sound. Had the warranty company come out and replace the motor, thinking this was the issue, but we are still having the same sound. Everything runs great while the unit is on and runs quietly and efficiently. The only issue is when the fan stops, there is a banging noise. Any thoughts?

Aug 07, 2018
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Moving Filter NEW
by: Anonymous

If this is happening INSIDE the house, it's probably NOT the condenser fan (on the unit OUTSIDE the house). Don't call anyone for help before you do this.

Take off your air filter at the return(s). Cycle the system through off/on a couple times. No clunk? Your issue is your air filter falling back down after the system shuts down.

Do one or more of the following:
1) Replace with less restrictive air filter
2) Tape or otherwise hold the air filter in the high position so it wont fall.

Done!

Jun 17, 2018
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loud noise when A/C cuts on NEW
by: Anonymous

I have also had this problem ever since replacing my unit with a Comfort Maker. It wakes me up at night every time the unit cuts on, it's right next to my bed. My installer has actually replaced it under warranty & it was worse. Now he's wanting to try to install a solenoid, whatever that is. this is a big issue that needs to be resolved with the manufacturers of these units!

Jun 05, 2018
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Slamming Noise on return Plenum NEW
by: Anonymous

I’ve found that if you are experiencing a slamming noise and it’s coming from the return plenum, it may be due to the fact that the unit may be oversized. Your air handle may be asking for more air than what’s available inside the house. You will hear the return plenum being sucked in (when fan starts) then return to its normal shape once the system turns off. Not sure that there is a good way to fix it, however. I was able to place a heavy brick on mine to simulate the sucked in state and prevent the sheet metal from constantly changing shapes.

Dec 09, 2017
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HVAC start up noise NEW
by: Peter

Every time our brand new Trane Air Handler starts up it makes a loud bang. The Trane installer said it is because of the electrical connections firing; this would not happen with a gas unit. The unit we had to replace...ten years old...made no such noise. Nor has any other HVAC we owned did so. Very frustrating, wakes us up at night.
Ideas? Thank you.

Oct 11, 2017
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Train XR17 condenser "bang" on shut off NEW
by: Anonymous

Had a new Trane XR17 system installed Spring 2017. Right off I noticed a loud bang or slamming sound when the outside condenser unit shuts off. Could even hear it well away from inside the house. Had the installer out several times for the problem. He told me that it was normal for high efficiency units. I did not buy into that. Researched these forums. Found folks who said that torquing the condenser fan nuts was the fix. Had the installer call Trane to investigate and he said he had to talk with several reps to find one that agreed that there was a problem and torquing was the fix. Installer came back out to torque the fan nuts. THAT WAS THE FIX. No more problem. I hope this post helps others. Pretty disappointed in Trane.

Aug 20, 2017
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great forum NEW
by: Lyle

not sure my first comment took. MY THEORY IS WITH ALL THE SUCTION GOING THRU THE RETURN THE TIN VENT IS SUCKED IN, JUST LIKE A PERSON SUCKING IN THEIR DIAPHRAGM. WHEN THE UNIT SHUTS DOWN THE PRESSURE STOPS AND THE TIN POPS BACK OUT. DO I HAVE ANYONE BUYING INTO MY THEORY.

Aug 20, 2017
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GREAT FORUM NEW
by: LYLE

Our AC unit is 2 years old. Overtime it shuts down, a second or 2 after it makes a bang, like someone is slapping the side of the ductwork. It drives my wife crazy. I say there is nothing wrong. the filter is a one 10x20x1 located in the hallway. When attempting to remove it while the unit is on, the suction is very powerful. I also noticed that it has another filter 20x25x1 or something like that. I stood by the unit and waited for it to turn off. When it did, the thump didn't happen. But it did a minute later when I came up from the basement. MY THEORY IS BECAUSE OF THE PRESSURE, THE AIR DUCT IS ALIKE A DIAPHRAGM AND THE DUCT IS BEING SUCKED IN. WHEN THE PRESSURE STOPS OR UNIT TURNS UP, IT POPS BACK OUT AND MKES THE NOISE OF TIN POPPING OUT. DO I HAVE ANY TAKERS WHO BUY INTO THIS THEORY.

Aug 11, 2017
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. NEW
by: MekaLekaHiMekaHineyHo

If it is a single, loud bang or slam then it is the compressor. There is a broken part inside of it, probably a spring. It cannot be fixed since the compressor is a sealed unit. Replace the compressor if the sound bothers you. It is not an indication that the compressor is going to fail any time soon. It may or it may not.

Aug 04, 2017
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Loud bang outside unit when it shuts off NEW
by: Anonymous

I have had this problem for several weeks. Just had the tech out today. He said he has seen it two times and Trane says the condensor fan mounting bolts are not tight enough. He popped the top cap off. This fan mounts hanging from the grill. The four bolts appeared tight, but he torqued them down more. Problem solved. No more waking up to a loud bang. Also good to know it was nothing serious.

May 29, 2017
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loud banging when shutting off NEW
by: Anonymous

We have a new Trane AC unit that is terribly loud with this banging sound when shutting off... It cools nice but it is simply very unpleasant as the sound of this outside compressor unit is LOUD ..Some say expansion valve and company will not admit it is a problem..Wow never again will I buy a trane!!

Oct 27, 2016
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Fuktart NEW
by: Anonymous

Anything on this post after jul9 2016 is whack and should just be erased

Sep 28, 2016
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AC OUTSIDE COMPRESSORE NOISE NEW
by: Anonymous

Enough of these fools commenting on the "fan blades hitting .. Or mispositioned" causing this banging noise.. Its caused by the design of the expansion valve... Its terribly loud but this is how TRANE designed it... So stop with the silly comments about the fan hitting something will you!!!

Aug 15, 2016
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Shut off Banging NEW
by: John

3 ton trane,2006 model, recently same problem as many below in this forum. Never really paid attetion to it, because the system never failed, cools the house well, so Ill see what the problem is tomorrow and write back what I find.

Jul 23, 2016
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Outdoor Split Makes Drill NOise NEW
by: Awais

I am sick of the drilling noise of my outdoor, i filled its gase point upto 75 and technician said its because of low voltage, but i don't think it is due to low voltage, pleade help me. Thanks.

Jul 21, 2016
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A/C Compressor making load noise before cycles off NEW
by: Ben

I have the same problem with my unit, compressor is making load noise before cycling off. Worst thing is the fan shuts off first before compressor makes load noise then cycles off. I have my clamp meter and check the readings, and wow when it's making that load noise before cycling off it actually draws a lot of current from the normal 9 to 11 amps going up to 21 Amps before cycling off. I was very frustrated changing both compressor and fan motor with no luck, even discharging the gas freon and vacuum the system for 30 minutes. I charge the gas freon in the normal fashion with the gauge reading with clamp meter to monitor the normal operating current, and even i have done the charging of gas freon by weight as per amount of Kl indicated. Still no luck. I decided to change my other unit but, before I remove my outdoor unit, I done one thing that I was not anticipating that could help solve the problem. I clean the fins of the grill thoroughly with a pressure washer, and i have observe that the load noise of compressor before cycling off was already gone. The best part is the fan and compressor now cycles off at time, and cycles on same time also. I have my clamp meter and manifold gauge check during the cycle off period, the clamp meter reading is now normal with the current rating the same as indicated in the sticker of the unit, and also the manifold gauge is only reading until 100-110 psi when compressor is not working, and before the gauge reading goes beyond 150 over the limit of my gauge. I'm staying in Qatar and temperatures here now are reaching almost 50 degrees C.

I am just working on my unit myself and I am not an expert, my A/C installation experience and diagnose I just based it by reading forums, and youtube search.

Jul 18, 2016
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Loud banging noise when outside unit shuts off NEW
by: Anonymous

My 4 ton trane unit makes the same bang noise when the outside unit shuts off. Its alright in the morning hours but as the days goes on and the unit runs longer( I live in Texas) The louder the noise the noise gets. Its only a years old and i had a technician come out and he haid there was no problem. Help

Jul 17, 2016
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TRANE SCROLL COMPRESSORS NOT GOOD NEW
by: Anonymous

Thank you for sharing your experience.

My 5 ton TRANE central air conditioning system is demonstrating the same loud banging noise when it shuts off.

What is strange about my situation is that when I turn the system on, it goes thru its normal cycles for the first hour or two without making noise when it shuts off; then,as the day progresses, towards the evening,the noise starts coming on and it gets very loud. It sounds like metal banging against the inside wall of the compressor housing.

I have called "so called TRANE experts", I have heard all kind of excuses and stories but unfortunately none of it gets my problem resolved

Jul 09, 2016
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The cause of ac slamming noise NEW
by: Anonymous

The slamming noise you hear when your ac shuts off is caused by the condensing fan blade banging into the fan blade housing. The fan blade is out of adjustment on the motor shaft usually caused by loose tightening bolt/screw or the fan blade is out of balance due to constant centrifugal force an additional cause is that the fan motor mount screws are not equally tightened or there is a slight bend/warp to the air grille fan motor mount. The reason the fan blade doesn't scrape against the housing while running is due to the slight lifting of the blade in the housing when the fan motor is energized and the helicopter effect of the spinning blade. The instant kick of electricity to the fan motor when starting causes the armature (shaft) to jump when the electricity flows through the stators. This instant jump raises the fan blade up and clears the housing at the spot where it is scraping. The springy action of the air grille fan motor mount contributes to this lifting effect. The actual spot where the fan blade is "slamming" into the housing is in between the high point when running and the low point when the fan is at rest that is why the fan blade appears to move freely within the housing when the motor is off and no scraping or rubbing is noticed upon inspection of the fan blade. If the outside edge of the fan blade is rounded,it creates an arc on the end of the blade and the "slamming" takes place when the center point of this arc bangs into the fan housing on shut down where the fan blade suddenly drops down because electricity is no longer energizing the motor. When the fan is running the center point of the arc is above the contact point on the housing.
To fix the problem, check the fan blade on the motor shaft to see if it is loose and adjust it up or down on the shaft so that the center point of the arc on the end of the blade is above or below the contact point on the fan housing. Check fan blades for damage and balance, and inspect the air grille motor mount for bends or warps causing the fan motor to be out of level which causes the fan blade to be cocked in the fan housing.
Any way you look at it, the slamming noise is caused by the condensing fan blade slamming into the fan housing on shut down.

Jul 07, 2016
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Trane outside air handler NEW
by: Anonymous

My 1 year outside Trane unit also makes a loud bang noise when compressor cuts off. Is this normal. It was a lot quitter when we first purchased it.

Jun 24, 2016
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2011 American Standard Compressure Loud Clunk At shutdown NEW
by: John H

Seems this 3.5ton American Standard joins the loud Shut Down Club. This unit was always a little bang at shutdown since new. A/C man said I leaked Freon R22 about 2lbs been 5yrs.system was charged and clip light leak sealer added. Now makes a louder clunk on turn off. That all It Starts Quiet Runs Quiet Cools Perfect 5yrs now. Coil cleaned . Should I be concerned ?

Apr 18, 2016
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Air Handler (Rheem) noise on start up and shut down NEW
by: Anonymous

This noise is very loud, almost like a scrubbing sound, on start up and shut down that last for 3-5 seconds. Unit does it in all modes, A/C, Heat, and Fan only. A/C Tech it stumped, and I'm tired from all the sleepless nights!! Any thoughts??

Jan 31, 2016
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Solution Worked for banging noise!! NEW
by: Tony

There was a metallic bang every time my heat turned on and off. It was hard to believe that my thin light weight air filters could make that much noise. They are in the ceiling and I could see right through the metal grate and watch the filters lift and fall on the grate as the blower turned on and off. I used tape to hold the filters in place after cleaning the surface that the tape adheres to.
PROBLEM SOLVED!

Jul 02, 2015
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Annoyed NEW
by: Anonymous

My landlord had a new AC unit installed and it is next to the master bedroom, during the night when the unit starts or shut off it makes a very loud banging enough to wake you up. Very annoying and when we let him him know he says it is normal!!
I do not think so. Please help

Jun 22, 2015
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My Rheem making slamming sound when shutting off NEW
by: Oklahoma

I replaced the outside fan motor on my heat pump Friday but when the unit quits cycling & shuts down, there is a loud banging noise as before and the fan is still moving. Called technician to come back. The fan blades were checked & no visible signs of being loose or cracked. Very confusing!

Jun 07, 2015
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American Standard makes loud pop at shutdown NEW
by: scooter

I have a 1 year old 3.5 ton unit that makes a loud bang when cycling off. The fan blade and shaft was changed in November and seemed to fix the problem foe about 2 months. Same problem again HVAC guys now telling me this is typical of these big units. Anyone else have this issue? Trying to figure out how far to take this. The unit works fine heats and conditions great just this loud bang is annoying.

Thanks!

May 26, 2015
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May be simple fix NEW
by: Anonymous

I had the same problem and could have sworn there was a door in the ducts that was slamming shut-- that's what it sounded like. It was actually the filter in the intake vent. When the unit would turn on it would slam against the top and when it turned off it was drop back down. I added some duct tape to keep the filter from moving and it solved the problem.

Jan 29, 2015
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same noise...seizing sound BANG NEW
by: Anonymous

I have the same problem with my girlfriends outside ac unit. Right when it shuts off a loud bang but the blades keep spinning but it sounds like they hit a wall. What causes that noise?

Dec 31, 2014
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Heat thumpers at the end of cycle NEW
by: David

This was in fact.. The problem!! The bottom of the " Air Handler" is NOT reinforced like the sides. And was in fact on wooden blocks to allow for condensate pan and the air handler to have a "Dry Zone" or gap so condensate can collect. I personally found a small roll of tape then slide it between pan and air handler. It need s to be slightly larger than the gap to provide a " preload " or pressure on the bottom panel.

Dec 23, 2014
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solved but not ductwork NEW
by: Anonymous

My problem was the bottom pan on the inside of the furnace. It is only fastened with front clips so when the fan turned on the back of the bottom pan would lift up and bang down after it shuts off. If your furnace is up on blocks, reach under your furnace and see if you can lift the pan. Turn the furnace on and see if it is lifted when it runs. Not sure if this is by design but it also sucks in all kinds of dust an garbage from the floor defeating the purpose of the air filter.

Oct 09, 2014
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quick question NEW
by: Anonymous

My ac when it turns on makes the pipe in the bathroom have a banging noise for as long as the ac runs. What xan it be? It only stops when ac cycles off

Sep 30, 2014
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A/C slamiming noise NEW
by: Anonymous

I believe the noise being experienced is known as "Water Hammer" as experienced in steam pipes. I believe this happens when the expansion valve closes when the compressor is stopped. It would resonate in the Freon lines.

Jul 25, 2014
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American Standard Noise when it stops NEW
by: Joseph

I just got a new HVAC 5-Ton HVAC system with a 2 stage cooling and when the scroll compressor stops, it makes a loud BANG noise.

It's not the fan blade from what I can tell.

Is this normal?

Jul 25, 2014
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Jul 21, 2014
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Slamming Noise Solved NEW
by: Hoover

I too had the "slamming/banging" noise issue with both of my Carrier 24ABA336A310 units when the compressor motor stops. I replaced both motors and both start capacitors and still experienced the same problem. On a whim, I decided to order two new fan blade assemblies...even though I did not have any faith that this would fix the issue. However; upon replacing the fan blades, the slamming noise disappeared. I inspected the old blades and determined that where the motor shaft attaches, the old design had a flaw that allowed the "hub" to move ever so slightly, which produces the slamming noise when it stops spinning since it is amplified through the blades. The new fan assembly does not have the same design so it appears as though Carrier identified the problem.

Jul 17, 2014
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Lennox non existing tech support NEW
by: Tony

First--big THANKS to ANTHONY for at least pinpointing the possible cause of the banging that my 4 year old XC13 developed this summer. Yes, it is of the type that is:
a scroll compressor using r-410a refrigerant (high pressure refrigerant).
Calling that 1-800-9-lennox number (PAM/MARK) is like calling the ACA line for help. Nothing but denial about the ‘noise’ fact but that ‘THEY HAVE’ build 100's of thousands of these SCROLL compressors and have great success with these in the industry.
Neither would touch the NOISE ISSUE at all but did bad mouth all of the INTERNET sites that post the helpful info. YEP-that damn river with the lousy name.
I had called one of the $70 for a house visit service call outfits, but cancelled it when a cold spell hit here in Kansas and the banging stopped. I did not want the service guy to think that I had extra money to throw away. Retired and on fixed income makes you do that sort of thing each time.
Finding the informative explanation by Anthony makes me think that I just may wait and possibly call for service if the (very loud) banging resumes once the 90+ days return..
OR-call Lennox and demand a new-QUIET- one.
Vengeance is sweet--NO?
Again thanks for the post, Anthony!

Jun 26, 2014
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2011 goodman r410a outside unit makes a gas release sound then shuts off NEW
by: wolverine

Outside ac unit runs for a few seconds then cycles off with a ssssss sound can u help me

Jun 26, 2014
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2011 goodman r410a outside unit makes a gas release sound then shuts off NEW
by: wolverine

Outside ac unit runs for a few seconds then cycles off with a ssssss sound can u help me

May 08, 2014
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Unit NEW
by: Anonymous

Live in mobile home when air cuts off there is a noise for about 10 seconds coming from area where coil fins are just start last night

May 08, 2014
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Unit NEW
by: Anonymous

Live in mobile home when air cuts off there is a noise for about 10 seconds coming from area where coil fins are just start last night

Apr 30, 2014
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Same problem NEW
by: Anonymous

I am having this same knocking problem with my Westinghouse AC making a knocking noise.

Mar 14, 2014
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HAPPY CAMPER AGAIN NEW
by: Wombat /manila

HAPPY CAMPER NOW , {from Jan,16, 2014} Service tech came out, the next day ,checked out duct work under home, checked the heat pump and controls, and freon reading,he had to reset high temp cut out switch that had opened, everything checked good, he said it was possible the variable speed air handler could have caused problem but amperage reading was good. the technician just charged a service call and said if this was to occur again it was very possible it was the air handler motor that was the cause, he made a note of this and said to just call back that it was covered by the warranty. ** today is march 14,2014. and the unit has run perfectly since Jan. 16 with no problems, this seer 16 packaged heat pump from Rheem is proving its worth with very low electric bills and keeping us comfortable. Deer Hunter, manila, zip 32009.

Feb 10, 2014
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Goodman thump noise NEW
by: Topper

My new Goodman unit makes a chunk or thump noise when the fan shuts down. It does this when it is just the fan operating (no AC or heat). I have put my hand on the handler unit at several different places and feel the thump the most on the direct-drive fan motor. Not so what the root cause is.

Jan 16, 2014
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NOT A HAPPY CAMPER NEW
by: Zip 32009

Installed New 5/3/12 RHEEM model RQRM package heat pump seer 16 . has made loud slamming sound winter or summer when shutting off since it was installed. Unit just quit 1/15/14. compressor an condenser fan quit. odd smell in house when it quit. turned on emergency heat and called dealer 1/16/14. the temp is 29 out side and 72 nice inside, waiting for repairman while electric meter does double time with strip heat. I am wondering if all this slamming was a defect in unit, that lead to such an early failure. My old coleman evcon ran 5 years before a part broke. Factory should pay the labor for this not just for the bad part on such a new and very expensive heat pump. Zip 32009. MANILA

Dec 08, 2013
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No answers? NEW
by: Anonymous

All the comments and not an answer one.

Aug 21, 2013
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Noise in AC when shutting down NEW
by: kikoman

I heard a clunky noise upon shutdown of AC unit downstairs. In winter it would be a different noise (compression and release of sides of duct work). A man came to the house fixed it. Said it was the duct work. Cheap fix. But now I hear it again and assume it is the same noise. Will call tomorrow just to be safe.

Aug 21, 2013
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Apr 20, 2012
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knocking sound 3 min. after shutdown. NEW
by: Wishin I had a Trane

I have a 2006 Rheem 3 ton split heat pump. Last fall I started to notice a Knocking sound occasionally after shut down. Now in cooling it happens after every cycle. I tryed just the blower and it doesn't happen. It's coming from the condensor and traveling on the lineset. Here's what I found so far: the unit cycles off, the indoor fan runs and additional 90 seconds, then within 60 seconds I hear a fast knocking sound that slowly deminishes in speed and volume.The duration is about 10 - 15 seconds. I haven't checked r22 pressures. superheat or subcool yet.
It is a scroll but, I'm thinking check valve in outdoor txv. Anyone ever heard of these symptoms?

Apr 20, 2012
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Mar 24, 2012
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god awful pounding in a/c
by: Dotty

My a/c is 21yrs. old. Even when the a/c is off I get this loud pounding in the walls and attic when the temperature changes from hot to cold. This happens even in winter. The new thing is a rumble of air rushes through the a/c vents. If I didn't know better I swear the house is haunted. Please Help.

Feb 23, 2012
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Both noises
by: Anthony in VA

Hey fellas,

This is in response to both noises (loud shut down & blower noise like hitting the wall). Seems like all these problems are with fairly new to not that old equipment. Chances are they are all scroll compressors using r-410a refrigerant(high pressure refrigerant).

It is known in the industry that on shut sown sometimes scroll compressors are noisy due to pressure equalization(this would make the knocking rattle sound). Sometimes it will make the metal noise which may be noise coming from the compressor plates (found in scroll compressors.

This noise is not supposed to happen, but it does with the occasional system. Other factors may be involved. Maybe it was a system replacement and they re-used the old line-set which was not exactly the right size, or too long. I have some units running perfectly for years with the noise.

The air handler noise bang on the wall on start up. Is this a variable speed fan coil? If yes, thats probably what you hear. On start up those motors start by rotating back and forth to find the right frequency before they ramp up speed.

If you hear that excessively the DC hub may be starting to go bad. If it is not variable speed and a psc motor, you may want to check to see if the blower wheel (squirrel cage) is in good shape. good luck!

Feb 03, 2012
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air blower hitting sound
by: Anonymous

Our heating unit is about 6 yrs old. Whenever it starts to work, the motor seems starting nicely, however, I would hear the air blower has this delayed response, then feels like something hitting the wall, for about 40 times. The hitting sound will go away after that. It seems worse when it starts from real cold temp and the hitting times shorten when temp is closer. Does anyone know why this is happening?

Jan 25, 2012
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I have the same problem
by: Anonymous

I have the same problem.When A/C or heat pume turns off, I can hear a fairly loud slamming noise in the house. like "Peng,Peng, Peng,..."It lasts about 5-20s. It sounds like a metal flap slamming shut, some way like the noise comes frome water tube.

Oct 17, 2011
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Slamming noise with subsequent rattle
by: Anonymous

We have the same "slamming" shut off noise problem followed by a rattle on one of the two Carrier model 24ACA air conditioning units that we installed in 2007 to cool our three story townhouse in Houston, TX.

I don't remember them making that much noise at first. They cool nicely but it has become difficult to work in the room located closest to the central air units because of the jarring noise.

We have had them serviced several times without much improvement and have replaced the compressor on one unit with no significant change. Is it also probable that the vibrations from the "slamming" is causing the central ac unit to rattle more as the problem continues?

Jul 18, 2011
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Indoor Fan Abrupt Sound
by: Dadan

I'm not 100 percent sure where the abrupt sound came from. It could be within the motor or a sound that is surrounding the motor. The sound could be cause by the motor force air.

You could check you duct system or even you filter. You could also make sure nothing is blocking the air flow.

You could also look at the indoor unit and ask someone to turn ON and OFF the indoor fan (without touching the indoor unit).

I hope the info help

Dadan

Jul 18, 2011
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INSIDE UNIT NOISE
by: Anonymous

I have a new outside/inside a/c unit, I am hearing the abrupt stop noise inside. When I called the folks who put the new unit in they said it was probably the blower in the furnace which was not replaced and is not under warranty (GRRRR).

I don't know if it is a problem or just a noise, only hear it when the unit cuts off and I hear it on the inside, any thoughts

Jul 15, 2011
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Me two
by: Anonymous

My Heil 5000 condensing unit when it shuts down makes a mid pitch metallic sound like the fan has hit the cage but it doesn't and just spins down normally. It's coming from somewhere else but I can't figure it out. Could it be the contactors?

Jul 14, 2011
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same issue
by: Anonymous

I have the same sound and think it has always been there. when starting, it sounds fine. but when shutting down, it's like a loud abrupt stop.

Jul 06, 2011
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Metallic sound at Shutdown
by: Anonymous

Mee too.a Heil 5000

Jun 04, 2011
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Exact same noise from 2yr old unit
by: Kathleen

Our 3 yr old Rheem makes the exact same noise only when the compressor shuts down. The unit cools nicely and is very energy efficient (great for living in South Florida), but it makes a lot of noise outside when shutting down. It's a loud clunking sound. It wasn't doing this the first year we used it.

Apr 26, 2011
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Compressor sound
by: Dadan

I understand once the central ac shut down, the compressor tries to equalize the pressure between the low and high side. In Heat Pump, it sounds somewhat like "Hissss" as it tries to equalize.

I'm guessing from your information. As the compressor shut down, the compressor piston in motion stops abruptly due to high pressure within the copper line.

The refrigerant pressure is great than the in motion piston to rotates. So, as the power cut from the compressor. It take within or less than a second to stop, due to the high pressure. Shawn, does this happen regularly, or it happen at specific outdoor temperature? As it stops abruptly, does it shake the condenser unit? I’m trying to see the big picture.

If my information is correct, there is reason why your central ac unit has higher pressure than normal. There could be airflow restriction across the evaporator coil (dirty filter), condenser coil needs to be clean or I’m just bark at the wrong tree.

I’m not sure what to tell you guys. I will pay close attention to different types of compressor sound next time.

Dadan

Apr 26, 2011
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Same here
by: Different Shawn

I have two four year old four ton units. Both central ac units seem to run fine and cool very well. When new they shut down/turned off without any unusual noise. Now they both have a clunking sound or as Ryan said a seizing sound.

To me it seems as if the compressors shuts down abruptly/instantly when power is cut instead of may be one or two more revolutions of the compressor as it slows to a stop.

Apr 14, 2011
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Slamming Sound
by: Shawn

Hey, Ryan.

If a compressor made a metallic noise, I doubt it would cool your house. Is this a Heat Pump unit or just regular central ac unit?

You could look around your compressor and checks for copper to metal object contact. If a vibration producing this metallic noise, you should hear when the compressor start.

I'm not sure if a spring form the bottom of the compressor with vibration is producing this sound.

Apr 14, 2011
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I have same problem
by: Ryan

Our outdoor a/c compressor unit runs fine.... same as always... cooling is excellent, but from the first day of use this year it makes a loud, short, sharp, metallic noise right as it shuts off.

It's not exactly like metal slamming something, but more like a seizing sound.

At first I thought it was the fan motor seizing on shutoff and the metallic noise was vibration from the fan blades coming to a sudden stop.

But upon inspection the fan keeps spinning freely as it should and the noise is definitely coming from the compressor.

Thanks!

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