What type of noise Mys? Fan squeal, buzzing, etc.... in most cases it'll probably be the fan gummed up with dust - best would be to remove the PSU from the case first -> with the power unplugged try sticking thin screwdriver into one of the fan exhaust holes to stop the fan blades from turning, then give it a good blast of compressed air; not too much though (and make sure there is no moisture in the air supply)
If out of warranty, maybe open the PSU top and without touching anything inside, try blow the dust off the fan from the inside out. I dab of electrical oil on the fan bearing shaft should do the trick to keep it running quietly for a while.