I think it's probably more common than people think, it's just quiet enough for a long time that most people don't notice it unless they're trying hard enough to notice it.

I suspect that's what happened to me at least. I think I actually did have it earlier in retrospect, it was just so quiet it was easy to ignore.

For the past four years though? Not so much. I can train my brain to ignore it still, but I need to be pretty focused on other things and/or cover it up a bit with ocean noises or be in a room with ambient noises or something. In a quiet room and I'm not really focused on something else, though, it's quite obvious and can get kind of loud at times.