I had the same problem since purchasing my 1984 300SD. Granted, I don't have the same model as your C class, but perhaps my experience may prove useful.

I mentioned that I had a "creaking" sound when hitting bumps, and my independent shop suggested silicone grease at the bottom edge of the inside door panel. Remembering from previous posts here that Lexol Vinylex contained silicone, I decided to detail all of my MB-Tex interior using Vinylex and terry cloth towels and diaper type cloths. I treated the rubber weather strips, as well as the entire door skins, all over, using liberal amounts of Vinylex.

Anyway, the car seems much quieter overall, and the squeaking is 95% gone!! I can only assume that the Vinylex "fed" the dry vinyl, making it heavier, improving its sound insulating qualities, and getting rid of the squeak by adding some lubrication.

I would estimate that I used half the bottle of Vinylex, so don't be afraid to apply, and reapply, if it looks like you need to. Even if this doesn't cure your problem, your door skins will look better Nix this idea if your panels have leather or cloth on them, though...

__________________
Bob Roe
Lehigh Valley PA USA
1973 Olds 88, 1972 MB 280SE, 1978 Datsun 280Z, 1971 Ford T-Bird, 1972 Olds 88, 1983 Nissan Sentra, 1985 Sentra, 1973 230.6, 1990 Acura Integra, 1991 Volvo 940GLE wagon, 1983 300SD, 1984 300SD, 1995 Subaru Legacy L wagon, 2002 Mountaineer, 1991 300TE wagon, 2008 Murano, 2007 R320CDI 4Matic 52K, some Hyundai, 2008 BMW 535xi wagon, all gone... currently
2007 Honda Odyssey Touring, 2014 E350 4matic