70stang - I have this same humming sound in my home. I hear it at night when I go to bed but I do not hear in the morning or during the day. I live in a rural location far away from any industrial or mechanical area. I have gone to different rooms and the sound is there. At first I thought it might be a car idling at my neighbors but it wasn't nor do I hear anything when I step outside.
My wife does not hear the noise but could be I am more sensitive to low frequencies. I too, am at a loss to explain this.