I once had to deal with a venue that had some decorative fountain / waterfall that was powered by a large pump. When that thing was running there so much voltage coming back to the sound system you'd swear the amp was physically dancing not just buzzing...

Maintenance guy finally got new pump and the problem went away.

Originally Posted By: XenonMan
The hair dryer example in the link above is an interesting thought. I wonder if there are some devices in your home that do a similar thing with changing voltage in a similar way. A multi-speed blender, an eco friendly refrigerator or even a heat pump that varies speed based on load could be a culprit. Since the problem seems to be related to the time of year maybe your AC or heating system is causing an issue.