Had some time over the weekend to start tackling the noise reduction project on my 420SEL.
The door seals all seem fine (quite flexible and snug); but nonetheless, I tried reconditioning them with some silicone to see if it made any difference. In a nutshell, it didn't.
Next step: replace the seals.
If that fails, I guess it's time time invest in some Dynamat...
1991 560SEL-Euro (145,000 miles)
1989 420SEL (Retired at 325,000 miles)