Its interesting to hear so many similar stories, as others have said, I thought I was crazy! I have heard this damned intemittent humming for about 3 years now.... at night when I am trying to get to sleep. I was absolutely sure that it was something in the enviornment BUT I have recently been to Africa, in the middle of the bush where there is NOTHING electronic or otherwise... and there it was!
I wish I know the answer to it all.