I also had a stall mat in my RTV1100 which worked great and was cut so it fit tight and would not move when dumping the box.
Wrapped my exhaust with exhaust wrap that everyone uses for cars. It was the 2" wide stuff.
Valve adjustment might help quiet the engine noises. Set to .005-.007 th clearance on both intake and exhaust.
Loose pieces like the safety bar for the hoist may be causing noise, remove it and see it that makes any difference.
Any attachments like trailer hitches can be a rattle that transmits throughout the machine.
Finally, I found both the 900 and 1100 noisy because of the diesel engine but soon got to ignoring it.
You will never make it as quiet as a gas engine no matter what you do to the machine!
Ear plugs for the humans also works very well.
On the plus side they sip fuel and run for ever.
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