The brief
The A6 was squealing on every cold start, a high-pitched chirp from the front of the engine the moment it fired. It would settle after the first few minutes once the engine warmed through, then screech again whenever the aircon kicked in under load.
That pattern is the classic ageing-belt signature. Cold rubber slips, warm rubber holds, hot rubber under a sudden load slips again. The belt is no longer gripping the pulleys the way it should, and slipping rubber squeals.
The fact that it changed with temperature and load, rather than being a constant noise, is what pointed at the belt rather than a seized pulley or a failing bearing.
The diagnosis
We took the belt off the engine for a proper look. The ribbed face was glazed, a hard shiny surface where it should have a slight matte grip, with fine cracking starting across the ribs. A belt in that state has lost the friction it depends on.
We checked the tensioner too. It had developed a small wobble on its bearing, which means it was no longer holding the belt against the pulleys with a steady, even force. The idler pulley still spun smoothly, but it was on the same age clock as everything else.
The right call on an accessory belt drive is the whole set: belt, tensioner, idler pulley. A fresh belt onto a wobbly tensioner just slips again, and you are back.
The work
Released the belt tension at the tensioner, then removed the old belt and the failed tensioner.
In went new VAG-spec parts: a new tensioner, a new idler pulley, and a fresh belt routed correctly through the pulley path. Routing matters; a belt run the wrong way around an idler wears out fast and can throw itself off.
Re-tensioned to the spec figure, not by feel. Too loose and it slips, too tight and it overloads the bearings in the alternator and the other accessories.
Then a warm-up cycle with the major loads applied, aircon on and electrical load up, to confirm the belt ran silently across the rev range under real demand.
The outcome
No cold-start squeal. No screech under aircon load. A silent belt drive across the whole rev range.
The A6 went home with the front of the engine quiet again and the slip path closed off. For the owner, the immediate win is the noise gone, no more chirp announcing the car on a cold morning. The longer-term win is reliability: a worn accessory belt that finally lets go can take out the charging and the cooling in one go and strand the car.
Doing the full set while the symptoms were still just a squeal means that whole failure mode is off the table for a long stretch.