The brief
The A4 was vibrating at idle hard enough that the owner could feel it through the seat. And there was a clear clunk whenever he shifted between Drive and Reverse.
Both are signs that the drivetrain was no longer being held the way it should. Engine and gearbox mounts do two jobs at once: hold the drivetrain in place, and absorb its vibration so it does not reach the cabin. When the rubber in those mounts collapses, both jobs fail.
The engine shakes the cabin at idle because the vibration is not being damped, and it lurches when you swap from Drive to Reverse because there is slack for it to move into, and the slack runs out with a clunk.
The diagnosis
On the lift, we pry-tested each mount in turn. The two engine mounts had collapsed rubber, one of them showing visible cracking. The gearbox mount was not yet collapsed, but it had measurable shift under load.
Replacing two and leaving the third would leave the drivetrain still mismatched: one corner held firmly on a new mount, another corner still slack on an old one, which makes the car feel worse, not better, and the still-old mount fails soon enough anyway. So the full set was the right call, all of them done together.
The work
Supported the engine and the gearbox on jacks, then removed each failed mount in turn. In went new VAG-spec replacements across the set, every fastener retorqued to the workshop manual values.
Before letting the engine's weight settle onto the new mounts, we checked the drivetrain was sitting in the correct alignment, because a drivetrain set down crooked puts uneven load on the fresh mounts and wears them early. Only once it was sitting right did the engine load come back onto them.
The outcome
The cabin smooth at idle, no buzz through the seat. No clunk on the Drive-to-Reverse shift. No vibration through the steering.
The A4 went home with the drivetrain held the way it was supposed to be. For the owner, that means a car that feels refined again at a standstill, and gear selection that happens quietly instead of with a thud.
Doing the full set rather than the worst two means the drivetrain is properly isolated all round, and this is not a job that comes back in a few months when the one old mount you left finally goes.
