I am in Las Vegas visiting my son. I Know, I know "what happens in Vegas stays in Vegas" but Every time his A/C comes on the drains in the apartment start thumping. I really don't know the particulars, but this I do know;the building is an eight unit building with one central A/C unit. I have my theories as to the cause but all may be way out there. 1, is they have connected the condensate line to the wasted line and while the A/C unit starts up vibration is heard. The noise seems to fade after a few minutes, while the A/C unit keeps running. I have been a life time fan of TOH and Ask TOH and I think I remember Rich T. talk about this once but don't remember the outcome (it's a age thing).