Noise from the unit outside

(Ca)

Every time the central ac unit kicks on it make a trouble noise like a crashing sound. Most of the time it is really loud sometimes not so loud, but it will always make the noise. My central ac unit is on the side of my house and is seating on cement.

We thought cause its not bolted down to the cement that it was rattling but were not sure. Filters are changed once a month and we wash out the unit often. We had a person that owns his own air condition company put it in but has no comment on why it makes noise, but would like to change the unit out for a small one. not going to happen. I have no problem having someone come out and look at it and fix it. I just don't what to have to pay someone to come out and tell me that their's noting wrong or tell me it is something that I could have fixed myself.