I know i'm digging up an old thread here, but i just wanted to thank Qhorin for sharing the sound clip.
I've been searching on and off for the past few months for sound clips of a bad intel fan on the usual tech forums and everyone just keeps telling me to check if the cables are in the way.
My fan makes that grinding noise when I shut down the computer. It sounds fine when I start up, and after a while there's a subtle rattling noise that's louder at lower speeds. Thankfully the uncertainty of whether the fan is going to die is over.