Quoted:
they make muffler silencers for atv's........ supposed to cut nois by almost 50%....... maybe try one of them?
I dont know how well they would work on a genny but they do work on an ATV. I bought the Hushpower brand. While riding my Sportsman ATV I could hear my buddies engine (both 400 cc's) over the top of mine and the wind noise, despite the fact that he was riding behind me. If they were both idling, you couldnt tell mine was running. I was pretty impressed with that, 50% is a fairly accurate estimate. That said, I would go with a cheap car muffler on a genny setup. Most of the gain was at low to medium throttle, at high RPM the air intake makes up a lot of the engine noise so I suspect it would be less effective on a gen set that runs pretty much full throttle.
If you install one of these on an ATV you quickly learn what they mean by "mechanical noise", as that is most of what you hear afterwards. That and the air intake.
I've conisdered rigging up something for my Coleman Powermate 5K. I can hear that sucker from inside the house when its running 150' away. Good thing I dont have close neighbors.