Ignoramus1606 wrote:


If only it was about combustion air. This is an air cooled genset that
requires a huge quantity of cooling air to cool the cylinder fins.

http://igor.chudov.com/tmp/onan/Diesel/

Oh boy.
The usual treatment is a serpentine air path lined with absorbent
material. You might try a box made of cement board with a serpentine
path (lined with rigid fiberglass insulation) in the front and back for
cooling air. I don't believe Sonex, Soundown or lead in an open box is
going to buy you much. The fix may end up to be a larger piece of
property and heavier electrical cable.

Kevin Gallimore