I'm not sure I'd do this. If the fan spins slower/quieter, you can cause it to overheat. An over heated PSU is NOT a good thing. Before doing this, I would look into replacing the fan with a better one. If you can find one that moves a lot of air, but is quieter, you have achieved what you wanted without having to increase temps.
I think he is talking about a case fan connected to a 4 pin molex on the power supply. If it is his power supply fan, then no, you don't want to do this.