i know how to make less noise with your generator.....buy a honda eu2000 and get rid of the noisy beast.
Or try and find an 1800 rpm generator. these are much quieter than the standard 3600 rpm geni.
I would love a honda EU, but i don't have the money. It's on the wishlist..
From what i've read, most noisy generators are not significantly quieter with a better muffler because the engine and generator itself makes a lot of noise.
Distance and Shielding both help. The problem with shielding is that most small engines are air cooled requiring lots of air flow. Putting the geni in an insulated box would make it much quieter, but without the proper precautions the engine would soon overheat.
Quis custodiet ipsos custodes (who will watch the watchmen?)