something in computer making a sound

I suspect it's a fan.  Strange thing is, it's not a sound I can easily describe.  The best description is, it sounds like some fan is vibrating... not a "whirrrrrr" or "buzzzzzzzzz" or anything like that, and the sound is one you have to really listen for in order to hear -- it's extremely subtle.

The way it sounded, I thought maybe one of the CDs in my CD-ROM drives was lightly, and very quietly, scraping the tray in rotation.  That's the best way I can describe the sound.  But the drives are empty, so I know it's not them.

It doesn't sound like a "holy shit something's fucking broken!" sound, but is it possible that it's deceptively quiet?

I've got an annoying sound as well :/

I strongly suspect it's one of my fans, but I can see nothing that could generate the noise (except the actual fans, I suppose) and it's impossible to pinpoint the noise.

Jamming the fans, eh?  Is there any chance of them being damaged in the process?