My wife and I have noticed a loud bang (can hear it anywhere in the whole house) every time our AC kicks on, or shuts down. It is a relatively new sound (just noticed it this spring). I'm pretty sure it was not making this sound ever in the past. This is different than the audible click that I've always heard near the thermostat when the system turns on and off.
Any ideas what is making the loud bang, and how to fix it?

Is the ac working OK otherwise? Loud bangs can come from several sources. Fan, compressor, wiring issues, capacitor issues, refridgerant issues.
Being it is at start and stop I would guess something with your compressor is the issue.

Other than the loud bang, it seems to work just fine. It maintains a temperature nicely. Turns on quickly. Easily distributes cold air around the house. Etc.

I would have a tech come look at it. That way they can hear the bang and try to diagnos from there.
There could be multiple things going on..

Does it do it in heat mode?
Try with ac and heat off, and turn tstat to fan/fan only setting, does it still make the big bang?

The bang you hear is the compressor motor hitting the side of the can it is in. The actual motor is inside of the sealed can that you see. The motor sits on four springs. That's why you don't feel much vibration when it is running. One of the springs has broken.

Now when the motor starts or stops it hits the side of the can. Nothing you can do about it. May run for a day or may run for 5 years or more. Mine has been doing that for about 5 years.

If you have a dirty return air filter you may be hearing the duct work popping. If it's a sound only on startup and shut down, with the info you gave it sounds like your return duct is popping.

If he can hear it anywhere in the house,I would guess the sound is coming through the duct work,, my thoughts also T-Top but ,maybe not

The bang you hear is the compressor motor hitting the side of the can it is in. The actual motor is inside of the sealed can that you see. The motor sits on four springs. That's why you don't feel much vibration when it is running. One of the springs has broken.

Now when the motor starts or stops it hits the side of the can. Nothing you can do about it. May run for a day or may run for 5 years or more. Mine has been doing that for about 5 years.


Most compressors sit on rubber rings in the middle of the condenser not springs. The rings are replaceable. And no the compressor can't just hit the side of the unit on 98% of most units.
I do wonder if the sound is coming from the outside or inside. I was tired from a long week and did not think to ask that.

((most compressors sit on rubber rings in the middle of the condensor not springs. the rings are replaceable. and no the compressor can't just hit the side of the unit on 98% of most units.
i do wonder if the sound is coming from the outside or inside. i was tired from a long week and did not think to ask that. ))

All tin can style motor compressors have internal spring mounts to support the motor/pump assembly. There are many reasons the manufactures started using spring mounts over the past 50 years but the main reason was to absorbe the motor starting torque and tp prevent internal damage to the internally attached pumping assembly.

Exploded view of compressor below

Air Conditioning and Refrigeration - Google Books (http://books.google.com/books?id=Sbu_X1b1s-oC&pg=PA208&lpg=PA208&dq=compressor+internal+spring+mounting&source=bl&ots=s8p9-muFiz&sig=BCor9Y9h-5vGBBZ7QUXceg3yv34&hl=en&ei=32ZrSsTBA5WANvnDtPkG&sa=X&oi=book_result&ct=result&resnum=4)


See item 7 below

http://202.56.127.90/nacg/EnggRec/EngineeringRecommendation16.pdf


And well answered by someone other than me.

The compressor you see is actually the outer shell, inside of which is a refrigerant pump (ie:compressor) which is on internal support springs. When the unit turns off, the momentum of the internal parts can sometimes knock against the outside of the compressor case. The only "fix" would be a new compressor. In the real world, the fix is to live with it. It's generally not an indication of immenent compressor failure and most likely will run many more years with no problem. Note: if you put your hand on most compressors (even new unused ones) and give them a shake, you will hear that noise.

See I was so tired I was thinking about the compressor and not the insides of one...

I understand. Mr. Tired and myself go back a long way. LOL