I read several threads on this problem on here like this one, but I'm not convinced I have the exact same problem. I've got a 1st Gen, 2000 PreRunner, about 80k, all stock.
I noticed a clicking sound when I turned on the heater fan about a month ago, and thought there was frozen moisture inside the fan or maybe some fragments of plastic, dirt, or whatever. Guess I should have checked then, because this morning the problem was worse.
It doesn't squeak or squeal like some of the others I've read on here. It sounds like a piece of paper is rubbing against the fan, more like a brushing sound, or a loose wire fell down and gets caught intermittently in the blades. It happens at all speeds.
I'm going to start by cleaning it, but here's the weird thing, that I didn't see in any other posts on this: my outside air vent is suddenly harder to switch open or closed, either way.
Same problem, or something else?