I did the entire floor pan of mine in a Dynamat type of product from a company called Car Builders, I then went over that in a thick acoustic foam mat that they also sell.
http://www.carbuilders.com.au/
its made a reasonable amount of difference in terms of noise reduction but not as much as you would expect.
I've been told doing the doors makes more of a difference, so I will do them too at some point and also possibly the roof.