That's an old one there with a 230w supply. Besides a noisey fan wearing out inside the supply or a case fan now loaded with dust the one other thing that could make any sounds are the hard drive or audio alerts from the board's speaker. When was the last time you cleaned out the inside of the case with a can of compressed air?
s it like a buzzing, or a rasping sound? cuz, fro first to second, it could be a bad fan, or a TON of dirt in between your case fan and the fan gaurd