The brief
The A3 had been using oil between services, with leaks and spots around the engine, a rough idle, blue or white smoke from the exhaust, and a check engine light, plus the occasional whistling or hissing from the bay. He brought it in. That whole list points at the oil separator, the part of the engine's breather system that catches the oil mist the engine breathes out and drains it back to the sump, sending the cleaned vapour back to the intake. A diaphragm inside it regulates the crankcase pressure. When that diaphragm fails and the housing cracks, the pressure isn't regulated, which is the rough idle and the whistling vacuum leak, and oil mist gets pulled straight through into the intake instead of being caught, which is the oil consumption, the leaks and the smoke. All of it traces back to that one part.
The diagnosis
On the lift the leak was confirmed at the oil separator, the housing cracked and the diaphragm inside failed, with oil tracking down and oil found in the intake side. The codes pointed at the crankcase breather. That's a replacement. The separator is a sealed assembly, you don't patch a cracked housing or a torn diaphragm, so the unit gets changed and the PCV system reset.
The work
The cracked oil separator came off, and a new genuine Audi-spec unit went in with a fresh diaphragm and seals. The breather hoses were reseated with new clamps so the whole path was sealed again, the oil that had tracked down cleaned off, the stored fault codes cleared, and a short adaptation cycle run on the scanner so the engine relearned its idle with the breather working properly. A road test confirmed the idle had settled, the smoke was gone, and there was nothing leaking.
The outcome
Steady idle, no smoke, oil consumption back to normal, no whistling, and the check engine light out after a drive cycle. The A3 went home with the PCV system regulating cleanly and the engine bay dry. A failed oil separator quietly drinks oil, fouls the intake and roughens the idle the longer it's run, so changing the unit and resetting the system sorted the lot.