So we all piled in the car and went down to Downers Grove for lunch at Fuddrucker's and a trip to Fry's to pick up a replacement power supply for my computer, since I'm rather tired of it shutting down on me for no apparent reason. I found the replacement supply on sale as advertised, so I saved about $25 on it. And I've now popped it in to replace the old power supply.
It turns out, though, that the power supply fan was not the source of the noise that I've been hearing. The noise is coming from the fan on the video card.
*sigh* I wonder if that's what's causing the unscheduled shutdowns. I guess I'll replace that some time soon.