I'm hearing a whirring sound coming from my computer. It speeds up and slows down, but starts only after my computer has been on for a few minutes. If I sort of rock the casing and let the computer bang gently on my desk, the sound stops briefly (figured this out accidentally, I'm not the type to beat my computer). I doubt it's the hard drive, since I har no clicking or anything like that, and my computer has suffered no slowdown. Does this sound like it could be my fan? if so, is there an easy fix for this? Thanks.