The brief
The A4 had developed bad vibration through the cabin and you could see the engine move around more than it should. He brought it in, the right call, hardened or bad engine mounts only get worse and they can stress the parts around them. 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 hardens, sags and cracks, and once it does the drivetrain moves around more than it's meant to: the vibration comes straight through, the engine note climbs in the cabin, and the slack lets it knock against its stops. Mounts that have gone together produce exactly that, 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 hardened and sunk with cracked rubber, the play obvious under load. 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 vibration 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 A4 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.
