Imac Whistling noise My computer has this annoying whistle sound that goes and comes back.  Does someone have an idea what it might be?  Do I need to take it to the apple shop? 

2.93 GHz Intel Core i7
12 GB 1333 MHz DDR3
ATI Radeon HD 5750 1024 MB
OS X 10.8.2 (12C60)
The motherboard (PRAM) needed resetting by turning off the computer and then restarting while pressing Cmd, Opt, P and R. The noise is gone.