For the gasoline powered generators, you can buy an after-market muffler which will be a lot quieter.
For the propane or nat-gas ones, I dont know. As it would have to fit inside the housing.
Take the cover off and look. Ask the company you bought it from, or hit the 'net, or auto parts store.
We ran a heavy duty cable underground to our shed, so that we can run one of them way out there, and not listen to noise. The other little 4KW one we use on the back porch, porch has a roof.
Thats why we prefer the gasoline ones, they are a bit heavy, but you can use a ramp to get them into the back of the pickup, and take them around to friends and family.