From...
https://celicahobby.com/ubbthreads/showflat.php?Number=16172...
go to your local discount store, Target, Walmart, Rite-ade, Thriftys or whatever... pickup a box of green felt sticky tabs for $1.49 (these are also called Velvet Feet). They are most commonly stuck under items that sit on glass or work tables and keep them from scratching up the furniture when they slide around the tops.
Anyways, take a few of these felt 'feet' and stick them under the door handles from the outside... this will absorb the vibration from your stereo or driving.
These are also great for placing under other areas that rattle... like plastics or latches.