Dimension goes "BOING"

I have a Dimension 2200 with a couple of years of good wear on it, but lately it has started making a persistent, loud "BOING" sound at odd times (through the speakers), stopped only by rebooting. It usually happens when I have several programs open (often including Adobe Photoshop CS2, a space hog). Sometimes the sound is "TSSUUUP!" (the sound of a Klingon shoulder weapon firing, if I recall). I can't identify a pattern, although it often starts when I'm in Word and use a macro (such as CTL-C to copy), although that's not consistent.

I'm running Windows XP Home with 1 GIG RAM. I admit to being a tinkerer who loves to try out odd programs I read about in computer magazines, so it is entirely possible I have a little program running somewhere that is causing this. I've tried simplifying things but haven't found the culprit, if there is one.

Also, if I turn off my speakers when either noise begins, I don't hear it and the computer seems to function normally. I usually reboot, however, just because I assume the repeated sound indicates trouble somewhere.

Any suggestions on how to deal with this?