According to TheKidd :

What you may want to try is building an enclosure that will house the
generator. The enclosure would be insulated, to dampen the noise.

The enclosure would have to be large enough to have at least a few inches of
room all around the generator. You'd also want to leave open one side of the
enclosure so that the exhaust had a place to go and also to prevent a fire.
You could then staple either insulation on the walls and cieling of the
enclosure or find egg-shell type sound proofing material to use. It will
not dampen the sound totally but could help.

To amplify a bit mo a recommended approach is to build a small enclosure,
with a couple of inches open at the bottom (all the way around), and a vent
at the top. Then line the enclosure with insulation. Make sure you have
SEVERAL inches of clearance all the way around the unit, and the bottom and
top vents are clear.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.