The brief
The GLC250 had a knocking sound while driving, worst over bumps and on gear changes. A knock doesn't always come from the undercarriage, sometimes it's the drivetrain mounts knocking against their stops. The team put it on the lift to be sure. He brought it in. The engine and gearbox sit on rubber mounts that hold their weight and absorb the shake, keeping it out of the cabin. As the rubber ages it sags and cracks, and once it does the drivetrain moves around more than it's meant to, so the slack lets it knock against its stops over bumps and on a shift. On a car this size and weight the mounts work hard and tend to go together, and left long enough the loose drivetrain stresses other parts, so they need changing as a set.
The diagnosis
On the lift the suspension was checked clean, and each drivetrain mount got a pry-test. The engine mount and the gearbox mount had sunk visibly with cracked rubber, the play obvious under load, which is exactly the knock. 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 knock was gone.
The outcome
Smooth at idle, no clatter on shifts or over bumps, no vibration through the cabin, and the engine note back to normal. The GLC250 went home with the drivetrain held properly again. Engine and gearbox 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.
