Originally Posted by grat

It is the secondary airpump. It's a fan that is located against the firewall at the rear of the right engine bank. It blows air into the exhaust system on startup and at idle when the engine is cold. The fan is fairly expensive to replace and the bearings only last so long. There is a posting on the A6 board where someone took it apart and replaced the bearings but it sounded like the fix was short lived. It only impacts the pollution control system and will not cause a problem with the engines normal function. You may get a check engine light if the air flow drops very low or the fan quits all together but again it won't stop the car. They can run for a long time making noise and still operate.

fj..