The brief
The GLC250 had picked up a heavy vibration through the cabin and the engine had got noticeably louder. That's a familiar one on these a few years in, the engine and gearbox are heavy and they wear the mounts hard over time. The drivetrain rests on rubber mounts that hold its weight and soak up the shake. The rubber ages, sags and cracks, and once a few of them have gone the engine and gearbox move around more than they should, so the vibration comes straight through to the cabin and the engine note climbs as the shake feeds into the body. On a car this heavy the mounts tend to tire together, and leaving it lets the loose drivetrain stress other parts, so the fix is a fresh set.
The diagnosis
On the lift each mount got a pry-test. The engine mount and the gearbox mount had sunk visibly with cracked, perished rubber. The whole set of drivetrain mounts was tired at the same time, just what you'd expect for the weight and the age. When they go together, you replace them together, a single fresh mount next to sagging ones only takes a beating, so the call was the full set.
The work
The engine and gearbox were lifted onto transmission jacks to take their weight, and each tired mount came off one by one. New genuine Mercedes-spec mounts went in across the set, every fastener torqued to the manual figures. With it all bolted up the drivetrain was checked sitting square in the bay before the engine was lowered onto the fresh mounts. A road test confirmed the heavy vibration and the loud engine note were both gone.
The outcome
Smooth at idle, no clatter on shifts or over bumps, no vibration through the cabin, and the engine note back where it belongs. The GLC250 went home with the drivetrain properly supported again. Engine mounts wear as a set on a car this heavy and they drag the whole car's refinement down with them, so doing the full set in one go reset it rather than leaving the next sagging mount to chase down later.