You may want to check 2 sources for soundproofing...1) Local Stereo Installation Store... many that specialize in high end systems have special soundproofing material and it can be installed in many ways... there is a plastic film type material that you place on the inner door panel frame - very easy install - cut trim to size and reinstall the door panels... this same material can be used in other locations..rear hatch for example... you can get the material from:
In a similar fashion there are also stick on dense pads that are used for soundproofing and can be placed on the interior door panels (metal) to absorb noise (this used vs. the spray on technique mentioned below)...I have seen these use on high end stereo systems and even available at auto parts mail giant:
2) Companies that do rustproofing (Ziebart) also have soundproofing qualities... for example when you have a panel with rustproofing sprayed on it there is much less noise transmitted through the panel ...I have removed many door panels over the years and have seen thick black "stuff" on the inner door skins (metal portions)and this deadens the sound substaintialy...you can get a similar effect by purchasing a rubberized undercoat spray product sold at local auto parts stores and remove the inner door trim panels and spray the inside of the door making sure not to block any of the drain holes. Companies such as Ziebart do this without removing the trim panels because they use the drain holes and drill other small holes to access these areas.
The above will substantially reduce the road noise heard inside the cabin... probably more than the newer ML's have.
3) Forgot one - you can update the door seals to the newest double door seals... search for this in this forum and the other informative ML forum at:
Hope this helps - Frank G.