Yes...the exhaust is horribly noisy. I did check the Kolpin. The problem is that when you put the box in the dump position there is not much room left to put an additional muffler. The only good option is a complete replacement with a bigger better muffler. That is proving difficult and may entale building a new muffler on our own?
For certain John Deere needs to come up with a solution to this problem. I am starting to hear tons of bad comments about the noise. The machine is great otherwise
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.