Hi.

Since I had alot of rust in the regular places
I had a good look at what MB did to lessen
the noise in the cabin.

The underside of the chassis is coated in a 1/4"
thick rubber under coating,which surely cuts sound.
When removing the floor mats you'll notice that
there is some sort of pad covering the metal
floor.
This,in its own right is sound proofing,unfortunately
it doesn't extend to the top of the dash,thus not
covering the whole fire wall.

If you have a leak you have to remove this stuff
because 9 out of 10 you have rust/rot under it.
I know I did.
Since you removed it and fixed the rust or you didn't
find any rust (lucky you),you'll need to replace it.
You could get a new pad from MB or you could use
the stuff that sound competitors use(dynamat or the
like,not cheap ),going this route will let you add the
sound insulation higher on the fire wall.

They also sell dynamat hood pads.
You could go as far as to add this stuff to the
firewall in the engine bay.
Cutting the noise-music-sound even more.

Just remember,if you have a turbo to add more
layers,to compensate for the extra heat.

Louis.