You have turned off everything that you know of and you still hear this hum/buz?
Did you turn off the main in the panel?
I know this sounds dumb but, have you tried placing a glass with your ear against it to the ceiling and walls to isolate the location of the noise?
I would stand in the living room and have someone turn of the main and see if it goes away.
Now for something really wierd, how old is the house? Or have you had any work done in this room or the room above it? Maybe there is something that was left behind that is causing the noise.( I know, it was a stretch)
If all else fails... I would try an Exercist! (kidding)