The brief
The A6 had developed heavy vibration through the cabin and a loud engine note that hadn't been there before. Those are the signs of worn engine mountings. The engine and gearbox sit on rubber mounts that take their weight and dampen the shake. As the rubber ages it sags and cracks, and once it does the drivetrain can move around more than it's meant to. That extra movement is the heavy vibration coming through and the louder engine note as the shake transmits straight into the body. Mounts that have sagged together produce exactly that, and left long enough the drivetrain knocking about stresses other components, so they need changing.
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, 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 Audi-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 A6 went home with the drivetrain held properly again. Engine 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.