Never experienced this one before -
Recently a very faint sound started coming through my computer's speakers. It sounds like an idling two-stroke motor - putt putt putt putt - incessantly. It stops when I either unplug the speakers from the jack or put my GPU idle by stopping Folding@Home.
A temporary fix was to plug the speakers in to the case jack and that stopped the noise while GPU is going full tilt.
Any idea of what could be causing this? I've updated both GPU driver and motherboard - problem persists.