I have a 550W eXtreme Power PSU and I was constantly experiencing jet-takeoff noise (OK, I know I'm exaggerating but you could hear it clearly 2 rooms away). Today I finally had enough and I decided to take it apart.
To my surprise, the PSU contains a standard 120mm fan which runs at high RPM, which is the source of the near-intolerable noise. I decided to take the chance and replace it with a Cooler Master 120mm silent fan and it worked like a charm.
My only worry is that the new fan won't cool enough and that the PSU will start overheating. To test if everything was going fine I ran Hellgate London for an hour (I have an OCed C2D and a X1950pro) to maximise power consumption and the air that was coming out of the PSU was barely lukewarm and the PSU was only slightly above room temp to the touch.
I only hope it will stay that way as this seems a cheap and permanent solution (it voids the warranty, though).