They are really loud but you can buy a better muffler such as one for an older lawn tractor with single cylinder at your farm store or Rural king to knock down the noise. They are really loud when under a load since you hear intake as well as exhaust noise.
I sold mine for $150 a few years ago and bought a used Honda off my neighbor. It's much more quiet. The muffler on the Honda is almost as large as the gas tank on big green monster.
Also, on the green monster, the stator is supported on the plug end by the plastic housing and a copper bushing. Mine melted. You can get a bearing conversion for them, but the plastic housing must still be good to install the bearing into it. If you plan to keep it, check the bushing and lubricate it.
Those of you with experience with these generators, how long do they run on a tank? It has a 1 gallon tank. Im considering getting my friend whos a welder to fab me a larger aluminum tank. Something has to be done about the muffler. The 18 year old me would have welcomed the noise, the 29 year old me, not so much.