There's two things that can couse that one your fan has A ton of dust on it try cleaning it. Two your fan could be going bad and need to be replaced.

They sell the's can's of air you can buy at most eltronic stores that should do the trick. I forget what there called though.

Try cleaning it first. If that doesn't work try this. Go into the Control Panel, Then Audio HQ, Then EAX Control Panel, If Chronos or Reverb are on at all turn them off or click All Effects Off...