What Philip says is all true.
A company called Noisekiller Acoustics makes a range of materials. They have a dynamat-type stick-on material and also a heavy-ish foam backed rubber material which can be cut to shape and laid under carpets, etc. this material is flexible engought to shape around trans tunnels, etc.
If the air vents make a big difference, and if in that sense you would be quite happy with the refinement of the cabin if the air vents could always remain shut, then your problem is (obviously, I suppose) noise transmission through the vent system. The way to tackle that would be by lining various parts of the vent system with sound absorbing materials. They should also be fire resistant, of course. A sound absorbing material is a material with a sound-porous surface, and a layer below the surface which absorbs and breaks up the air vibrations. This might take the form of a thin layer of perforated but closed-cell foam over a layer of open-cell or even reticulated foam.
On some noisy semi-race engines I have had some success by just lining the induction air pipes with a fire-retardent reticulated foam.
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