Every time my a/c goes on, something makes a BZZZT! sound for second. It seemed to start when my contractor reduced the speed of my blower motor. I searched on the internet, and I read about contact chatter which could apparently have a number of causes: dirty relay contacts, bad capacitor, bad transformer, or blower motor needing oil. Only I don't seem to hear this buzz when I'm standing right next to the air handler when it goes on, and I also don't seem to hear it when I'm standing outside next to the condenser when it goes on. I hear it when I'm in a room between the air handler and the condenser. It seems to happen every time, and it's loud enough to wake me up sometimes. I have also heard this noise when the a/c for a neighboring condo goes on, but I don't think I've ever heard it with my unit before now. I hung out in the attic between these units because some of the electrical goes through there, and I didn't hear it there either! WTF?