The brief
The GLC250 had developed heavy vibration through the cabin, the kind that says the engine isn't being held the way it should. On a big, heavy SUV like this the engine and its mounts take more stress than on a lighter car, so worn engine mounts are a common consideration as one of these ages. The engine and gearbox sit on rubber mounts that take their weight and soak up the shake. As the rubber ages and the weight of the drivetrain works it, it sags and cracks, and once it does the drivetrain moves around more than it's meant to. That extra movement is the vibration coming through, the clunk as it rocks on a gear change, and the harsher engine note. Mounts that have sagged together produce exactly that, and on a vehicle this size they only get worse faster.
The diagnosis
On the lift each mount got a pry-test. The engine mount and the gearbox mount had sunk visibly with cracked rubber. 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, especially on a heavy car, 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 heavy vibration and the harshness were gone.
The outcome
Smooth at idle, no clunk on shifts or over bumps, no heavy vibration through the cabin, and the engine note back to normal. The GLC250 went home with the drivetrain held properly again. Engine mounts wear as a set, and on a big SUV they take the comfort down with them, so doing the whole set together resets the lot rather than chasing the next sagging one a few months later.