sounds likeyou either have a fan going bad in the PC (CPU or power supply usually, though sometimes there are others in the case) or your hard disk is getting ready to give up the ghost.

I would try to isolate which of the ones it is... CPU fans are readily available, if it's the power supply then you need to replace the power supply unit (spares are not too expensive)... if it's not a fan then I would recomment backing up all your data in a hurry

__________________
-Toast-
"It takes a lot of brains to enjoy satire, humor and wit - but none to be offended by them." -- Johnny Hart ("BC", cartoonist, 2000)
"Analyzing humor is like dissecting a frog: Nobody really enjoys it and the frog generally dies as a result." -- E. B. White