The usual suspects in noisy operation are the fans or a hard drive starting to go. The fans need be inspected and cleaned(brush them off) and general cleaning of the system case. Check *all* fans. A HD on the other hand either to due to heat build-up or simply starting fail, may cause its bearing or the beingginning of a failed drive before it crashes. Serios, you bet, but if heat, in either case remove case cover and blow a household fan into it and see if the noise abates or it appears to lessen. A "stuffed" system cases causes inefficent operation of exhausting the heat build-up and just plays havoc with everything, fans included. So inspect the fans try to locate noise source with attentive ears. There are other possibilites, but these should be looked at first. If you've done recent h/w upgrades or such, backtrack and look at that as well.

good luck -----Willy Happy