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?