I built my boat from scratch, and while getting it running I started doing things to quiet the salon down. Like most here, my motor is right under the deck in the center of the salon.
First thing: Added sound down to underside of deck between deck frames. Result, slight improvement.
Next: Carpet over deck, including big motor hatch, but not over little access hatch. Result, big improvement.
Next: Weather strip on hatch covers. Result, big improvement.
Next: Seal up penetrations from engine room to salon/cabin environment. Results, improvement in stateroom, but no change in salon. Salon has basically no holes between ER and salon by design.
Next: And this is a strange one!!! Engine throttle cable was zip tied to cable saddles screwed to deck frames. Strange noise came through deck. Added rubber bushings to saddles, noise gone. Same noise I hear at the throttle lever. Injection pump has some wicked vibes that run all the way up the cable.
I think the biggest thing is sealing holes between ER and people spaces. Easier on some boats than others. The sound down was expensive and hard to install, and did less than the carpet and weatherstrips.
Some diesels make lots of noise at air intake, that can be handled at the source.
Now if I can get rid of my prop noise over 23kts without buying a $2500 prop.