Sometimes my computer begins to peep. Usually this happens when I run cpu-intensive programs.

Can it be that it is my power supply that is peeping?

I've put my ear against it at the back of my computer, where the air flows out and I can hear the peeping clearly. There are people that say that it is my CPU but I can't distinguish, the sounds is too high for that.
The sounds is also audible when I go into BIOS.

I've disconnected power on all my IDE-devices and I didn't hear the sound in BIOS, then I reconnected power on one of my cdrom drives and the sound reappeared.

Also when I run SETI, the sound is audible but the sound always goes away after a period of time, a minute or 2 or 3.

What is causing this sound and is there something wrong? It has always been like this. I've got an Aopen HX-45 case with the Aopen power supply of 235W. Here in Belgium, Europe, voltage settings are 220V.

Abit BE6-II
P 3 550 katmai
TNT2 M64