[gwilli]

Noise isolation is tricky. The first check is to make sure your mounts are still resilient and not dried and hard. Make sure there are no short circuits around them for vibration from the engine to transmit to the hull or engine enclosure (directly touching). Low frequency because of long wave length is difficult to isolate as it travels through barriers but closed cell foam or preferably closed cell foam with lead-vinyl backing will provide some high frequency absorption. Same kind of stuff is used for machine enclosures. Closed cell is necessary so it doesn''t get saturated with oil. Also very important to ensure a tight fit of enclosure adjoining inhabited spaces. A tiny crack can let through a lot of noise. Of course you need air for the diesel but this should come from spaces indirectly adjoining or baffled from liveable areas. Good luck.