I have heard the same hum for about 2 years now. I live in Muskoka Ontario Canada. Noticed it after I moved here. I have found myself outside in the middle of the night in -20 temps and 2 feet of snow trying to figure it out cause it was driving me nuts. My wife hears it now and so does my brother but most people don't know what I am talking about.

One just can't pinpoint it , it's all around. And annoying as hell.