I've had a few machines where the idler wheel on the drive belt would wear a burr on the metal shaft causing it to bind. It's mounted on a spring loaded arm, so every time it bound up for just a fraction of a second, it would cause the arm to jump and make a rattling noise. You can either replace the idler wheel if it's really worn, or check the steel shaft carefully and lightly file off any burr then polish it up with some very fine sandpaper. Not that hard to get to usually.
Mike Hawkins:smile: