There's not much room for insulation any more. My old Johnnies had combo rubber / foam pads glued to the inside of the cowl, that kept peeling off even after re-gluing them, probably due to oil contamination.
The short intake runners have to be open so irrespective of the remainder of the cowl, not much different than sitting under the hood of your car with no air filter nor air filter housing in place.
At idle rpms incl. trolling, the 4 stroke is as quiet as a car engine, at speed just as loud as anything else, after all they have a valve train and run higher RPMS, how can they be quieter vs. any 2 stroke? A different tone perhaps, but just as loud. Loud is loud. My Parker F225 buddy ran up his engine one day at the marina he never did before, whoa it was loud. Hard to tell on the water unless both boats are identical model.
At speed, the 2 strokes do fire all cylinders every rotation vs. every other rotation for the 4 strokers.
You can try throw away ear plugs like construction people use, the cheap foam ones they sell at the depot.
When my twin HPDIs idle at the marina, they are not intrusive at all, can carry on a normal tone conversation sitting right next to them, many do comment (the 4 stroke guys seem surprised, I wonder why?) as they are quiet, and considering there are two of them running not just one.
