I have to agree with lnxr0x. I had a computer that did that and it was driving me insane. I took the side of the case of and found that it was in fact a fan, but it like yours wouldn't do it all the time. And remember, it could be worse... It could be and evil server!!!!!!!! NOOOOO!!!!!!!!!! (if you don't understand that, its time to start watching the show a little closer)
Good luck