The disk activity (motor) is affecting your sound system in some way. Have you replaced the drive at all recently? There's not much you can do about this short of a logic board replacement or trying a different hard drive. Sound systems have filtering to prevent magnetic fields from things like hard drives from affecting the sound, but something might be slightly off on your machine. If it's still under warranty and it's really bothering you (and you haven't replace the hard drive), take it in for a repair.
Steve