[TAKTAK]

i thnik what your are encountering is what is known as 'coil whine' OR 'coil hum', which is where the coils on the motherboard/GFX card/PSU reverberate and make the sound that you are hearing...

some people stop the whine by putting a material onto the coil to stop the rerverberation (such as a resin) but most things come supplied with something allready on the coils (will look like hot melt glue)

but coil whine can occur in anything that has coils... ever heard a TV emmitting a high pitched noise? thats would be coil whine aswell...

the newer ATi cards are actually buggers for having the problem...

but to pinpoint where the noise is coming from exactly, use a rolled up newspaper on your ear and use the other end like a stethascope (i.e. take the side panel off and listen to the various components)... (sounds weird but it works) and this will allow you to determine whether it is the PSU or another component that is whining.