This may sound like a dumb idea, but have you tried other power outlets at your place, or maybe taken it to a friend's house to see if the problem is still present? I had a hum problem I couldn't figure at my old place, which mysteriously disappeared when I found a better outlet elsewhere in the house...of course, this was an old, old house, that should have been rededicated as "The Museum of Residential Electrical History" with exhibits ranging from knob & tube to silver romex to catgut and whale blubber, but that's another story...anyway..just a thought.
