I'm hearing speaker hum in a stereo system that I recently hooked up. I can't pinpoint when the hum begins, but I've identified that turning certain lights off or on in the same room eliminates the hum.
I've identified two light switches that have this effect – not all switches in the room do. When turning one of these switches on or off I sometimes hear a 'pop'. The hum does go away after an indeterminate length of time if I don't touch any lights, but I haven't identified what might be coming on or off (appliances elsewhere in the house, etc.) that would cause this.