If you look at my photos in our blog, from when the engine compartment was totally re-insulated, note the similarities to what Christian did for his boat. Worth noting also is his use of the word "suppression" rather than ellimination. There are vibrations and 'noise' from across a spectrum of sound to try and attenuate. I did not use the two layer foam with the "decoupling" layer, but kind of wish that I had.
Problem is not that our new engine is noisy (it is gratifyingly quieter than the old one) but that all diesels make a lot of noise.
Also, IF you have any small openings or cracks along the edges of the engine panels or front 'box', noise will leak out. Those vibrations/sounds just "want to be free"...
I have visited aboard new vessels with thick rubber gaskets around the access to the doors/openings to their engines, and this is likely why.
An example is the open structure behind the engine that communicates with the lazaretto and aft cabin or quarter berth paneling. We notice the difference when the large aft berth cushion is in place rather than out of the boat.
ALL the effort is worth while, but to some extent it will always be a "game of inches".
Aside: when I operate our club's dredge with a 250 hp Cat diesel, from inside the insulated cab, I and the other operators wear thick ear protection. Outside on the side deck the roar at 2000 rpm is rather impressive albeit tiring.
(Heat and exhaust are not the only waste products from these hard working engines -- also sound energy.)