I guess drugs would be the obvious answer....
I used a vibration dampening mat. It's very heavy. I also used it on the firewall and under the seats and on the front trunk and rear trunk floors. It definitely cut the sound down.
McMaster-Carr - Do a search on Vibration and select Vibration Control Compounds.
http://www.mcmaster.com/I got the cheapest one. I also bought a 5 gallon pail of the paint stuff to use inside the doors and the wheel wells, but haven't done it yet. I figure I've added 50 lbs to the car....
James