I used to think it was something boring a tunnel deep under ground, maybe aliens that would pop up somewhere and destroy the world. Ahhhhhhhhhhh. t`would make for a good film, don`t you think?
I hear the hum, it's usually late at night when all is quiet. I haven't got particularly good hearing so I don't really understand why I should hear it. It is a natural progressive noise to me which I accept as just the world turning. Indoor or outdoors.
I've heard of it but not heard it. I think we can all hear noises in our heads (IMO - blood rushing and so on) but this one doesn't appear to have any reason. I am glad I can't hear it, I hate extraneous noise.
The nearest I get is when next door put on their extractor fan, it sounds like Cape Canaveral when it's on full.