You need to be quite careful playing around inside a power supply. There are capacitors in there that still have a charge when the psu is unplugged. They can give you a real nasty jolt if you are clanking around in there with a screwdriver, not to mention ruining the psu. Watch what you are doing when replacing those fans. I've replaced psu fans with quieter Panaflos with no problems, but when you can afford it, try the Enermax or Vantec quiet psu's---well worth the investment.