Our NC33 from, er, 1982?, with a Ford Lehman 90HP engine, has two removable wheelhouse floor sections with a one inch(?) or thicker sound-absorbent fibreglass wool (?) mat held in place by sheets of perforated (what looks like) thin galvanised steel. (I cut through some of this sound insulation from the saloon into the engine compartment, to make a small hatch through which to tighten – or remove – the engine drive belts, and it took A LOT of cutting to get through the really thick sound absorbent section with a mini angle-grinder!)
Our plywood engine box beneath the wheelhouse floor doesn’t seem to have any lining at all. Nevertheless – although I haven’t measured it (sorry!) – our engine noise and vibration seem reasonable at all engine speeds ..and a LOT less noisy than the deafening high-revving Volvo in Tim Spall’s motor barge the Princess Matilda, judging by his TV programmes! Our engine noise and vibration seem similar to Skyler’s (is it?) on which Nauticat we went out to see the Tall Ships when the NA had that Nauticat event near Belfast a few years ago.
From many trips on many different kinds of large engine boats, I’d say that the NC33 noise is average to lower than many – or most – other boats I’ve navigated/driven/steered/skippered.
There are many reverberating wood-on-wood junctions where the floors and engine-box panels meet: I would apply self-adhesive rubber strips along all wood-on-wood mating surfaces – e.g; where the floor panels rest on their supports, ditto the engine-box panels – to absorb the ‘rattle’ of vibrating wooden panels. That may remove some low frequency annoying ‘throb’ or jitter.
You say “..The silencer is in good condition but can it be upgraded?..” We don’t have any kind of ‘silencer’ on our engine, just a steel elbow (which I replaced, as it had rusted) which conveys cooling water into the exhaust system, and then along a re-inforced rubber hose beneath the aft-cabin bed and out through a skin fitting. This is normal for many, or most, boats, so there’s no exhaust ‘baffle’ box to quieten the engine exhaust. Ours doesn’t seem noisy, but just burbles along as the exhaust noise is absorbed by the injected water. You could – theoretically – insert an exhaust ‘baffle’ or ‘muffler’ box, which cars and lorries have, but I doubt that it’d make much difference. (I’ve ridden in several oldish Rolls-Royces, and they are really pretty noisy compared with, say, modern Audis.)
I’d say seek out VIBRATION, such as where wood panels meet each other, including steps down to the aft cabin, door to aft toilet compartment, etc, and reduce vibration there by using rubber padding between all those surfaces.