There's a company called SoundDown (located in FL I think) that specializes in reducing noise levels from boat engine rooms. They sell a lead shield, foam backed product that does a tremendous job. Seems the real trick is to search out and eliminate all free air flow. They actually showed me with their equipment how much noise will travel through a 1/4" dia. hole.

I used them to insulate the engine room on a 72' Bertram Convertible I sold to a European client. Good people to deal with.

I'd do an internet search and contact them with your information.