We were experiencing a lot of noise inside the coach, from the undermount Onan 4000 gas genereator - waaay more noise than outside... what the heck? The tailpipe noise is minimal, even without a muffler and the generator itself is tolerable outside - but inside was a serious racket!
With propane heating and our solar setup, we don't use the generator for much more than the toaster, coffee maker and microwave. Our climate rarely calls for air conditioning and we're not full-timers, so it took me about five years to finally focus on the noise issue.
I can't really explain how I got to the conclusion but when I had my "Eureka" moment what I realized is, with the Onan 4000 being mounted to the frame, we were experienced vibration more than noise from the generator. The hollow plywood bench boxes were acting like loudspeakers, turning the generator vibration into full-throated sound!
I chose a Rockwool product called Comfortboard 80 which is a semi-rigid product, easy to cut and fit. This product absorbs most of the sound and has the added benefit of providing a thermal insulating value of R4.2/in.
