Agent86, i suggest you visit some of the Tech/Repair forums on other RV forums, especially RV.net's Tech Issues forum.
i've been reading questions/solutions like yours, for over 8 years, about quieting down generators like the Champion 3500.
there's a thread there, on Chinese generators, that is over 500 pages long with hundreds of posts about what you're asking.
and Zoe the Wonder Dog(R.I.P.)
'07 Roo 23SS pushing an '07 Chevy Avalanche
Equalizer WDH and Prodigy BC