Few options -
First as loply suggests, you want to determine the ultimate source of your noise. If it's the fan, then you can deal with that, if it's the power supply's electronics, then you might want to go shopping for a new PS (I'm using the Zalman ZM300A, and it's very awesome).
If you're ambitious and aren't afraid of electronic devices (and know what you're doing), then you could try a few things with the PS fan.
1. Replace the fan with a quieter one - i.e. a Panaflo.
2. Try those rubber mounts from Silicon Accoustics, I've heard great things about them.
3. If you're feeling particularly ambitious, you could run your power supply fan off of your fanbus. That would require a bit of wiring, but that way you'd be able to determine exactly what was causing your noise in the first place (i.e. turn off your PS fan for a little while and see if you still hear the noise). You could also throw in a switch for the power supply fan - turn it off if you want to leave your computer on at night while in standby mode.
4. If the noise is due to airflow (which it probably isn't, because you describe it as a high-pitched noise), you might consider doing a mod on the back of your power supply. Dremmel out the PS fan grille, round out all the edges (this is important! - it could get louder if you don't do this), and replace with a store-bought fan grille.
As for relocating the fan to outside the case - I do not recommend it. Reason being is that the airflow should be strongest near the heatsinks on the PS for best heat transfer. If you take the fan outside the PS, then it immediately becomes much less effective due to distance from the heatsinks and any resistance/pressure drops. You'd need a much stronger fan to do this, and that would likely make your system quite a bit noisier.
Any rate, if you've got the cash for a Zalman ZM300A, I'd say go for it. I've had nothing but good experiences. It's got one quiet fan (thermally controlled), and the only noise I hear is due to airflow. Otherwise, try a Panaflo, but make sure the PS stays cool enough, fires are no fun.
Hope this helps.