> (Don't confuse this with the world wide paranormal hum people claim to hear)
Taking this sort of random swipe at at people who experience this is kind of rude. Even though it's pretty much confirmed it's not a real audio signal coming from outside of a person's body, there isn't quite enough evidence it isn't a real auditory sensation for these people (instead of psychological or "paranormal" issue).
I do get the hum sometimes, usually happens when my shoulders are tense. Also usually only in one ear, which is why I was able to pretty quickly rule out an external source (though when it first happened, I did spend some time opening and closing windows, listening at ventilation ducts, trying to locate it).
Edit: I'll add it here that sometimes it is a real sound. We live a couple of km away from a harbor, and the sound we get to our place from an idling cruise liner is very similar to what my ears sometimes generate by themselves. It's really hard to tell them apart except by trying if it gets louder (and more high freq components) by opening a window. The point is, at least in an urban environment it can be pretty hard to tell if the hum is real or not.