I have the two seat version and did some work to see what I could do to quiet it down. The short story is that nothing helped much but I got the biggest drop by sealing the front bottom edge of the dump bed to prevent noise from having a direct path to the passenger compartment. This gave me about 1db decrease in sound. I was able to lower the sound by 1db. Post #6 in
this thread shows where I installed the foam. That and some of the other tricks to quiet the dump bed from rattling are the only things I might recommend.
I also installed sound deadening mat to the underside of most of the body panels. It was a lot of work and I only saw a 1db decrease in the sound. The full story of what I tried is
here.
There is just so much open area for sound to get up into the passenger area that I feel it's mostly useless to attack the noise down low. There is one thing I am thinking of trying and that is installing a eggshell type sound deadening foam to the bottom side of the roof. I had little success keeping noise out of the passenger compartment but maybe killing what hits the roof and preventing it from reflecting back would be more productive.