If you can get to the case easily , or is it a laptop?
anyway, listen around to locate where the sound is loudest.

most likely is the fan squealing - if the fan stops then that;s when things can overheat.

If you have pets or if it's never been cleaned inside that can cause the fan to bind & squeal.

http://duckduckgo.com/?q=cleaning+your+computer

the hard drive usually makes squeeks, clunks or thumps.