Another thing to look at is your speaker wiring. I had a similar problem with my setup and my circulator motor on my furnace. (hot water baseboard)

Every time the circulator would come on I'd get a POP from my rear speakers.
I had the wires for the rear speakers running through the floor, across the basement and back up throught he floor to the rear speakers at the back of the room.

The wires were running along a central beam in the basement which just happened to be very near the wiring for the furnace components. I simply moved the speaker wires a couple feet away from that wiring and haven't heard a pop since. (well over 5 years now)