In home depot, they have spray in foam insulation. I don't remember the brand.... Anyway, you can put this stuff in any small space and it will secure movement. and if you overfill the small space, you can cut it smooth.
Hell, cut it smooth, bondo over it and paint it. That will give it the original look, and it won't rattle at all. Just don't get it into the lock cylinder. those things are hell to clean out!