There are damping materials you can get for cars that works fairly well:
http://www.dynamat.com/automotive-and-transportation/car-audio/dynamat-xtreme/ (their website is pretty terrible..)
There are of course many others.
http://www.dynamat.com/automotive-and-transportation/car-audio/dynamat-xtreme/ (their website is pretty terrible..)
There are of course many others.