Gary, I just installed the 15kw Generac unit at my home. I don't find that it is loud at all. In fact, I'm impressed with how quit it is for an air cooled v-twin.
You are correct about the muffler. It's a very large automotive type muffler. I don't know how much research you've done on this unit, but the Generac V-Twin is one heck of an engine. Have you seen this unit in person?
While Generac can be tough to get information out of, they do offer a wealth of information on their website in pdf format.
I looked at almost everything out there, and ended up getting this one because it was the most cost effective. (and I can put the whole house on it.--HVAC is a geothermal heat-pump)
If you have any questions about this unit, feel free to e-mail me. After many repeated calls to many dealers, and Generac themselves, I think I've asked everything there is to ask. Hope this helps

-Ben

P.S. You can buy portable generators that run on NG/LP, but the fuel conversion runs the cost WAY above the gasoling models.