Diesel generator just a bit too loud - how to quiet it down?

There is no easy DIY way to quiet genset.
You can get it few % quieter by adding sound-absorbing mats into genset compartment, maybe couple less dB by getting upgraded muffler, but I bet lot of that noise comes from engine itself and I assume it is not inverter, meaning running constantly at 1200 rpm, or multiply.
I had the same issue with 3600W Generac, that even design for RV was noisily.
My solution was carrying Honda 3000EI in storage bay.
Not only drastic drop in noise, but also drastic drop in fuel consuption.