Best Discription I Can Give Is Whistle

by Robert M

This is a Lennox brand central air and heating unit installed in the attic. It has a metal strip you remove to install the ac filter, and slide into place.

When you install the filter and replace the metal
strip you get the noise at the cold air return in the living room ceiling. If I remove the strip the noise quits.

I tried adjusting the filter, but it is so difficult to work with because of it's design I can do much adjusting.

Any suggestions?


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Jul 04, 2019
We have a “whistle sound” too
by: C Greback

My husband noticed immediately and asked about & was told we needed more air ducts. No. Not going with that option. I will check the metal strip around the filter & see if he still hears it. His office is close to the unit too.

May 15, 2016
whistle NEW
by: Anonymous

When this whistle happened to me,I found the filter was not in place properly. Check to make sure it is slid in all the way.

Apr 07, 2012
Whistle Noise
by: Al

I also have a whistling noise coming from my indoor A/C unit. Every major component in the inside unit has been replaced. I noticed the noise after replacing the filters so I'm wondering if that could be the problem. I just had an A/C service technician check out the unit and he could not find anything wrong but could not explain the whistling noise. Anyone have any ideas??

Dec 20, 2010
Central Ac Whistle
by: Dadan

Hey, Bob

Have you found the solution?

I was thinking. On some central ac unit, there is two different fan speed for heating and cooling mode.

In cooling mode the air speed is higher then heating model. It possible that in cooling mode, it had always produce this sound. The reason is, when you open the metal strip. It didn't produce that sound. It basically, allow air into the system, beside the nearby return vent.

Or, you could verify this by block the other vent (not the 10 feet one) . If you block the distance vent and it produce higher whistle sound, we know it is the air velocity. Bob, if you do found the solution, come back here and tell us. We are curious.

Dec 02, 2010
Best discription I can give is a whistle
by: Bob

Okay, thanks for all your suggestions.I'll
try the fan on without the heat or air running.
I have a tendency to go with some type of obstruction, or a hole in the duct as you
Another thing that just crossed my mind is
the cold air return is probably not more then
10 feet from the unit. There is another return
in a hall way further away, and you don't hear
the noise.

Thanks for your help and time i'll
keeping trying different things until I find the culprit

Dec 01, 2010
Whistle Sound
by: Dadan

When you turn the thermostat to Fan On (without turning central air conditioner unit on.) you would hear it right?

The whistle sound came from air velocity. Look for small hole in between the return vent and the indoor blower. Yes, the flexible duct could be the problem. It could also be that the return vent, itself.

It could cause the whistle sound. Try to open the return vent from the ceiling and turn the thermostat to FAN ON position (without turning the central ac unit on). Does it produces the whistle sound? Sorry about the late reply, I have been too busying lately.

There could be some material is on top of the return vent. It could have cover with debris or other object

If the bent motor shaft cause the whistle, it should have produce the sound when it was put in. I'm guessing the technician though a huge rat could had scratch it back against the shaft.

Nov 29, 2010
Best discription I can give is a whistle
by: Robert

I have already tried the tape and making air
tight. It didn't change anything, it still
I can't give you a picture, because the unit
is in my 80 yr. old sister's house and I'm trying
to help her out.
She had a contractor out to work on it
last week and they told her the fan inside
had a bent shaft and that was causing the problem
so after replacing the fan and $450 later it
still has the whistle.
I'm certainly not an air condition tech, but
I know enough about mechanics that you start with
the simplest things first.
And no, I didn't try with the fan on the "on"
position, only in the "auto".
The filter is 1" thick and was put in by
the techs that worked on it. However I might try
a thinner filter.
I am also wondering if a kink or sharp bend in
the one flexible duct could be a culprit

Thanks for your suggetions and help

Nov 29, 2010
Air Leaks
by: Dadan

If you uploading a picture, it could help. It seems the whistle sound came from the air leaks into the filter door (metal strip). Is the metal strip air tight? If you turn the fan into ON position, it would still make the noise right?

Try this:

1.Put the metal strip back into the air handler unit.
2.Get piece of paper and run it around the metal strip to see if the air is leaking.
Here my thought on this.

Presuming the metal strip is the filter door.
When you fully open the filter door, it didn’t make noise. So try make the filter door air tight and see if it still make the noise.

Put a tape around the metal strip to make it air tight. It also possible the ac filter is to thick for your central air conditioning unit. But I doubt it.

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