The brief
The A6 was leaving coolant puddles overnight. Small but consistent, the kind of patch that tells you the system is losing fluid somewhere rather than just being slightly low.
A few other things came with it. The temperature gauge had been creeping higher than it used to on normal drives. There was a faint whine from the front of the engine, audible at idle if you listened for it. And the cabin heater was slow to warm up on cold mornings, which happens when the coolant level drops enough that there is not always a full charge of hot fluid going through the heater core.
Four signs, and they all converge on the same component: the water pump.
The diagnosis
We pressure-tested the cooling system to find exactly where it was losing fluid. The leak traced to the water pump shaft seal, the seal that keeps coolant on its side of the pump while the shaft spins. Once that seal weeps, coolant escapes around the shaft and runs down the front of the engine, which matched the dried residue we could see staining the bay.
Then we put a stethoscope on the pump with the engine running. The bearing inside had developed a roughness you could hear, and that was the source of the whine.
Two failures in one part: the seal and the bearing. Once a pump is at that stage, replacement is the right scope, not a reseal that would leave the worn bearing in place.
The work
Drained the cooling system into a clean catch, then released the drive belt section to clear access to the pump.
The failed water pump came out. On this engine it is an integrated module, pump and thermostat housing in one unit, so the whole assembly gets swapped together. A new VAG-spec replacement went in with a fresh seal.
Refilled with the correct coolant at the right ratio, then ran the bleed cycle to clear the air pockets that always trap in a modern cooling system after a refill. Topping up alone does not get them out; the system has to be bled the proper way or you end up with a hot spot and a gauge that wanders.
Last step was holding pressure on the system to confirm there were no weeps anywhere before the car left.
The outcome
No drips on the floor overnight. Temperature gauge steady in the normal band on a road test. No whine from the front of the engine. And the cabin heater warming up at the right point again, which is the quiet confirmation that the coolant level is full and circulating the way it should.
The A6 went home with the cooling system back to spec. For the owner, the practical win is straightforward: a car that holds its coolant, runs at the right temperature, and is not quietly heading toward an overheating event. A water pump left to fail completely can strand the car or, worse, cook the engine. Catching it at the puddle-and-whine stage kept it to a planned repair.