Electric whirring/buzzing noise in Mac Pro 2 x 3GHz Quad-Core Intel Xeon

Every 10 minutes exactly my Mac pro makes a strange electrical whirring buzzing type sound. its hard to describe, but it is about 1.5 seconds long and quite loud. It seems to happen even when the computer is sleeping but at a less frequent interval. Any ideas or suggestions as to how i can make it stop?