Really, my PC buzzes sometimes and only stops if I smack it on the side of the case. Any idea what it is? I'm pretty sure it's not a disk drive, because it happens with or without a CD in either drive.
I know whacking it can't be good for the computer, but it's really annoying.
I had this for ages before i worked out what was causing it (in my case anyway). It was the cable for the side panel fan - every time I took the side off to see what was making the noise I obviously disconnected the cable to remove the side panel with the fan on it! I was convinced it was a hard drive - because it didn't do it when the side was off (and it was on its side) I thought a drive was on it's way out and it only rattled when the case was upright.
I recommend starting it up with the case open and you should find the problem pretty quickly.
And yes, vacuum your case anyway, it could probably do with it unless you do it regularly.