The brief
The A4's aircon had gone weak, worst at idle. In slow traffic the cabin air would drift from cold to merely cool, which in Singapore is the difference between comfortable and not.
There was also a rattle. It came from the front of the engine bay, and it only showed up when the AC compressor engaged, which is a useful clue: a noise tied to one component switching on and off usually belongs to that component.
Weak cooling, worse at idle, plus a compressor-linked rattle. Three pointers, all at the compressor.
The diagnosis
A pressure check on the AC system showed it low on refrigerant, which tells you the system is losing it somewhere but not where. So we listened at the compressor with the engine running.
Two things. The clutch was hesitating to engage cleanly, not snapping in the way a healthy one does. And once it did pull in, there was a bearing rattle from inside the compressor body.
A worn clutch plus worn internal bearings is a compressor at the end of its life. And a compressor whose bearings are going has been shedding fine metal debris into the AC lines, so the job is not just the compressor: it is the compressor, a full flush of the lines and condenser to clear that debris, and a fresh receiver-drier to catch anything left behind.
The work
Recovered the remaining refrigerant into the reclaim machine first, since venting it is both bad practice and against the rules.
The failed compressor came off, along with its drive section. Then the flush: cleaning solvent run through the AC lines and the condenser to chase out the metal debris from the worn unit. Skip that and the new compressor inherits the old one's grit and fails inside a month.
A new Audi-spec compressor went on, with a fresh receiver-drier, both pre-charged with the correct refrigerant oil. Then the system gets vacuumed down to a deep pull, which boils off any moisture, and once that vacuum holds steady the loop is sealed and dry.
Refrigerant charged in by weight to the Audi figure, not by guess.
The outcome
Cold air at the vents at idle, the cabin staying cold in stop-start traffic. No rattle when the compressor engaged. The clutch grabbing cleanly every time.
The A4 went home with the aircon working the way it should. For the owner, the practical win is straightforward: a cold cabin within a minute of starting the car, which matters every single day in this climate.
And because the flush and the receiver-drier were done as part of the job rather than skipped, the new compressor should get its full service life.