My guess is CPU fan. But some Gateway powersupplies have fans that are too loud. Look for a higher quality power supply. Also, certain models have CPU fans that are load and bad (particularly K6 and other Slot 1) Only other choice is the case fan, but they are not usually noisy. All these fans are very low priced items made by the millions. Even new ones can easily fail. Luckily, the are cheap enough if you shop around. First, test the CPU fan by replacing it with a CoolerMaster or better. then look for an Antec, Sparkle, or other good quality power supply (many mentioned in other sections of Forums.