What kind of noise?
The sound from a fan, sound from a hard drive, and the whining of a capacitor are all distinct and easily distinguishable.
With Windows XP machines, if the indexing service is turned on, then often during idle you'll hear the hard drive reading, writing and seeking heavily, until you move the mouse to bring the system out of idle (at which point indexing will stop, and so the noise of the hard drive). You can turn this off by going into the Properties
of the drive in My Computer
and unchecking Allow indexing service to index this disk for fast file searching
. Phantom in particular, this may explain why your hard drive is making a lot of noise during idle.
FlyingWizard, a good way of finding out where the sound is coming from is to open the case, turn the system on, and just following it with your ear. Not to close to the fans though