Do not forget that after getting rid of the power supply noise, you will probably hear the other fans in the case that were previously "covered" by the louder one : CPU fan, case fans, if you have them, maybe video card fan (this last one was the loudest in my PC). The best solution is just to shut it off, as Maxgain said. Cleaning all the fans on a regular basis helps; internal dampening is also a good idea.
