I recently noticed that my PC was a lot quieter than it had been. I didn't think much of it until I reached around the back to plug in a USB pen drive and noticed it was extremeley hot. I cracked the case and discovered that the power supply fan had quit. Since it's a decommissioned workstation from another division of my employer, I easily located a spare and replaced it.

The moral of the story is this: if it sounds different, something has probably changed and it's worth a few minutes to take a look inside. In my example, I saved the motherboard from cooking and still have a nicely configured 'nix box to surf JL with!