Google tells me that this is called "The sound of impending doom"

It appears that my iPod has been sneaking around behind my back, and cheating on me with gravity. It seems to have either taken a tumble on one of its adulterous romps, or it has contracted some sort of sexually transmitted (computer) infection, and will now do nothing but futilely grind its harddrive, and use its sad face () to express its contrition.

Or it was gremlins.