Perhaps you've already considered this, but could it be caused by a fan or some other source of mechanical vibration in your house? Acoustic propagation within a house can be surprisingly complicated.
Some years ago I noticed a very strange hum in one corner of my basement. Thanks to funky acoustics, I couldn't identify the source for a while, but IIRC it ended up being a fan in another part of the house. I suspect most of the energy traveled through the house framing, and ended up resonating in the room I was in at the time.
Some years ago I noticed a very strange hum in one corner of my basement. Thanks to funky acoustics, I couldn't identify the source for a while, but IIRC it ended up being a fan in another part of the house. I suspect most of the energy traveled through the house framing, and ended up resonating in the room I was in at the time.