If it's idle and the drive light isn't flickering then I'd guess that's the fan.
Turn it off and clean the fan vents on the back/side from the outside with a soft brush. It that doesn't help you need to take the side off and brush them properly. As well as the rear and side ones there will be one on the CPU/Motherboard. You should be gentle with it, but if it's jammed up with fluff it will overheat and die.