Hello. For the last several months my wife and I noticed a low frequency humming sound in our home. At first this sound was simply "white noise" and not very noticeable unless you really tried to focus on it. However in the last couple of months the sound has been much more noticeable, particularly when all the TV's in the house are turned off. It's gotten to the point where many nights I have to go sleep on the couch with a fan turned on full blast. It's the only thing that masks the sound enough for me to sleep. Ear plugs are of no help. They seem to drown out every other noise except for the hum. Following are more details:

1) Can be heard in every room of the house.
2) The noise is pretty much constant but seems to pulsate/vary in intensity.
3) I've shut off the power and the sound is still there.
4) I've looked throughout the home including the attic and can't seem to locate the source.

Any ideas or feedback would be VERY MUCH appreciated. Thanks in advance.