It is radiated rf from the blower motor. This rf is not coming in on the line but is radiated directly to your system. Sometimes moving cables around or moving your preamp around could help but this is a real difficult problem to eliminate other than not running the blower when listening
Help on Electronic Problem
I have a fireplace in the room where I have my stereo and when the fireplace powers up the blower to dissipate heat from the fireplace, I hear static (sometimes loud static) coming through my speakers. I have a PS audio power purifier that I plug my system into. Yet I still hear the static (whether music is playing or not). What is this and what do I do?