The brief
The GLC250 had developed heavy vibration through the cabin and a loud engine note that wasn't there before. On a car this size and weight that's a common one a few years in, the engine and gearbox are heavy and they work the mounts hard. The drivetrain sits on rubber mounts that carry its weight and absorb the shake. As the rubber ages it sags and cracks, and once it does the engine and gearbox move around more than they're meant to, so the vibration comes straight through and the engine note gets louder as the shake transmits into the body. Mounts on a heavy car like this tend to go together, and left long enough the drivetrain knocking about 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 gearbox mount had sunk visibly with cracked rubber. All of the drivetrain mounts were tired at the same time, exactly what you'd expect on a car this weight at this age. 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 heavy vibration and the loud engine note were gone.
The outcome
Smooth at idle, no clatter 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, especially on a car this heavy, 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.