Before buying my 2017 p1000, I literally went to dealerships with a noise test as my top priority.... Kubota, Polaris, Kawasaki and Yamaha. The Polaris XP900 was the least noisy, Mule FX and P1000 were about equal, the others I dismissed in under 10 seconds when running.
The ability to change gears early on the Honda helps at low speeds but once adding hard roof and full windshields, the noise was back at Yamaha level. I added carpet using the expensive 3M spray glue but had to screw it on too because the black roof in the sun would soften the glue and caused major sagging, the carpet did help a bit as did car hood liner insulation from the srapyard placed under seats, but the only real solution is to drive slow and change gears early.
I get that these are essentially open air motorcycles but for the price I would have hoped for a little more effort from the engineers on this. It really does annoy just about everyone to some degree, a bit like loud music at a bar you have to shout over, no one likes it, everyone complains about it, there is a fairly simple solution, yet it continues...oh well.
I sold the 1000 and bought a near mint Japanese mini truck, once lifted and trail tires added, it will be no more than a third of the price of my 1000 and is street legal, has heat, AC, a decent bed and damn near silent, but I did like that pioneer.