We moved into a new house a year ago, when the AC unit shut down in the middle of the night(set at auto), we heard a loud banging like someone was trying to bang on doors or windows to get in. It scared the crap out of me. It last about 30 seconds. I thought after a few more times that it was with the fan unit outside but when I got down stairs to look out the window the unit was not even running and I could still hear the noise. I eliminated that as the souce. I look in my attig where the upstairs unit is located and there was only one pipe that ran behind the wall at my bedroom where I can hear it the loudest. It was secure and not loose. Is there something major wrong with the unit? This happens when it has been running for a while and shuts down whether it is ac or heat on? I can't sleep at night with this. Is it with the compress or or pressure what? The builder is giving me the run around and not sending someone to fix it. Is this a major or minor problem? I don't know much about this stuff. Please advise.

Check your air filters at the return air grills. restricted air flow on the return will have this symptom. Not cross breaking your square ducts when made can have the same effect.