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...
Lehigh Valley PA USA
73 Olds 88, 72 MB 280SE, 78 Datsun 280Z, 71 T-Bird, 72 Olds 88, 83 Nissan Sentra, 85 Sentra, 73 230.6, 91 Integra, 83 300SD, 91 Volvo 940GLE wagon, 84 300SD, 95 Subaru Legacy L wagon, 02 Mountaineer, 91 300TE, 08 Murano, 2007 R320CDI 4Matic 52K, some Hyundai, 2008 BMW 535xi wagon all gone... currently
2007 Honda Odyssey Touring, 2011 Honda Odyssey Touring