I have a (completely unscientific) theory that my tinnitus is a result of early exposure to CRT TV's and my brain trying to compensate for the noise. The reason I say that is that it's roughly in the same frequency band as the PAL horizontal refresh. It's been with me basically my whole life, since long before any real hearing damage would have set in anyway - I remember asking friends when I was 9 or 10 whether they could hear it too. It wouldn't surprise me if there was a window of opportunity when the brain is still plastic for these kind of "adaptations" to set in place.