[AChimp]

Run your computer with the cover off for a few days so that if the noise comes back, you have easy access to the insides to see where the sound is coming from.

Weird noises are usually fans, or cables that have fallen against fans. Sometimes in the process of opening up your case, you move stuff around and then the problem isn't obvious anymore.

If it is your hard drive, it could be that the drive was just thrashing a lot. This basically means the grinding sound you heard wasn't grinding at all, but the read/write heads moving back and forth more than usual (typically happens when you have a lot of fragmented data on your drive). Or your hard drive is dying.