For as long as I've had this PC (over a year), the main hard drive has made this intermittent scratching noise. It is some sort of Seagate 80GB (or 75GB, that's all it sees). Originally, I thought nothing of it, and went about my business. Then a month or so ago, I heard this horrible clicking, and the PC was locked up. Naturally, I hit the power switch in the back quickly, and everything was fine after that. There wasn't really enough time to tell if it was the HD or something else (like a fan) that had caused the situation, but for some reason I thought it was the HD. Now, either in truth or through paranoia, it seems that the scratching is more and more frequent. I've never noticed any data errors or other poor performance from the HD, but now that the paranoia is stuck in my head, the sound is really annoying. Here is a recording of it, although my mic isn't of great quality, so it's a bit hard to hear. The sound occurs three times during the recording.
It is never faster than once every one or two seconds, and sometimes it doesn't happen at all. However, it doesn't seem to correlate to any reading or writing. Fortunately, I have pretty much everything on it backed up, and a 160GB HD sitting unopened on a shelf, so if it does die, I should be back up fairly quickly, but I don't want to go swapping it until I have to.