I'd go with MBenz.
If you want the door to be more solid - you could add some extra carbon/tar insulators. They come in pieces of about 1 x 1 ft. I had that done on the W126 and the door sounds a little heavier than usual.
To be sure, run a magnet test and see if it sticks to your door. Don't use a too-powerful magnet otherwise it could attract through the paste/cement that was deployed if at all any repairs were dont to that door. If it doesnt stick, that means your door would have been HEAVILY pasted on.
Do make sure the inside panels are tucked in and bolted/clipped properly as it would make a difference on the sound if loose.
Then make sure too that the exterior panel is well clipped, for that too could make a difference when the door is shut.
126 tailed by a 203, 129 leading the pack.