The brief
The C300 had developed excessive vibration through the cabin, a knocking sound from the engine bay, and the gear changes had gone clunky, all signs that the drivetrain mounts have had it. The engine and gearbox sit on rubber mounts that hold their weight, keep them in place, and absorb the shake. As the rubber ages it sags and cracks, and once it does the drivetrain moves around more than it's meant to: the vibration comes straight through, the slack lets it knock against its stops, and on a shift the drivetrain rocks instead of staying planted, which is the clunk through the gearbox. Mounts that have gone together produce exactly that, and left long enough the loose drivetrain stresses other parts, so they need changing as a set.
The diagnosis
On the lift each mount got a pry-test. The engine mount and the transmission mount had sunk visibly with cracked rubber, the play obvious under load. All of the drivetrain mounts were tired at the same time. When they go together like that, you do them as a set, fitting one fresh mount next to a sagging one just loads the new one harder, so the call was the full set.
The work
The engine and gearbox were taken onto transmission jacks to take their weight, and each tired mount came off in turn. New genuine Mercedes-spec mounts went in across the set, every fastener torqued to the manual figures. With everything bolted up, the drivetrain was checked to be sitting square in the bay before the engine was let down to load onto the fresh mounts. A road test confirmed the vibration, the knock and the clunky shifts were all gone.
The outcome
Smooth at idle, no clatter on shifts or over bumps, no vibration through the cabin, and the gear changes back to normal. The C300 went home with the drivetrain held properly again. Engine and transmission mounts wear as a set and they take the rest of the drivetrain's refinement down with them, so doing the whole set together resets the lot rather than chasing the next sagging one a few months later.
