Do you hear it come from the Computer its self or the speakers? Even LCD's can cause the noise, most of the time thats normal, other times its a sign of something ready to go like a cam on the Mobo or Video card. When you launch a game the makes it buzz, Open the side and slowly stop any moving fan, and see if its a fan causing the buzz.
It could be alot of things, it could be a wire caught in a fan, coil whine from the video card, the LCD monitor, speakers and the list can go on. To get a better idea of what can be causing it is to try to pin point as best as possible exactly where the sound is coming from.