how to eliminate air-born RF

Sounds like it might be 60 Hz hum from a transformer of some kind, but that only occurs if there is something powered up. Any chance you might have low voltage halogen lights on a different circuit breaker? If you still hear it and the whole house mains breaker is off, you may want to get your ears checked--might also be tinnitis...

Assuming that Edesilva might be on the right track about a very strong electromagnetic field (EMF)expanding and contracting at 60hz frequency from a rogue external power equipment, your speaker cables can pick up that like a wire-loop induction. If your speakers are efficient enough it might be playing the hum all day long without your amp turnon. I can be wrong, but try disconnect your speaker cables and see if the hum is still playing.

I do hope I'm wrong in this intriguing Mooncrikit case. There were many health concerns raised by people who live near sources of EMF recorded by Mark A. Pinsky in his book "The EMF Book" 1995 Warner.

I corresponded with Mooncrikit briefly and from what i can gather, Edisilva seems to be on the right track. It is almost as if there is an external source for the vibration / hum. Only problem is, Mooncrikit has experienced this when the AC mains were disabled, meaning that the power to the entire house was not functional. As such, it looks like Philip's suggestion about an EM field could be coming into play here. I have experienced this problem to what most would consider a "scary" and very visible extent before, but not inside of a house.

As to try and narrow things down, how far is your house from a large pole transformer ? Do you have High Voltage power lines in close proximity to your house ? Is the AC feed into your house in very close proximity to this room ?

To be quite honest, this sounds rather baffling to say the least. Sean
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This is a real shot in the dark, but check to see if you have a thermostacially-controlled vent fan in the gable end of your house. When I moved into a house once I did not know one existed. I keep hearing a damn humm, but only in one room, and only on occasion. Now, if you have truly disconnected power from the main, my point is moot.

yes check your attic for a attic exhaust fan ours is quite noticeable if your near the wall where its mounted into. But killing the power to the entire house should turn its power supply off also? or possibly do you have any of those round attic vents they aren't powered by electricity they work through the heat rising it could need new bearings if you have any.