I've had this, and it was diagnosed as tinitus.
Like you it was a dull, industrial hum. The noise would come and go, at varying volumes, but was most noticable at night. One of the strange things that I had, was, that if I tapped the back of my head, the noise would momentarily dissapear, then return.
Doctors couldn't really do much, tried an anti-vertigo drug, with no effect.
It has eventually waned to a point where I don't even notice the occasional buzz that still happens.
Sorry to say, it's taken about four years for most of the symptoms to go.