There is a great way to reduce engine noise
to a quiet barely noticeable hum..by converting to electric propulsion
Oh I'm on a roll today.
Next someone will ask how to stop that smell of diesel
This was discussed some time back, i'm sure the thread can be found, but basically if you want to go cheap
(like not going electric
and DIY, go to the junkyard, and rip out the heavy rubber mats on the floor/firewall of cars, the more luxury model the better. Glue it to some super thin lightweight concrete board used for bathrooms, they have like 1/4in stuff, this will give you some fire protection and backing for the rubber,
and while your at it, put some aluminum
foil on the inside to keep the heat away too.
And of course always ensure the engine has good ventilation in/out, but not in/out of your habitable space. Just my $0.03