I have a 00 A8 4.2 quattro.
The car runs fine exept this strange noise that seems to come from the dash, it is very infrequent, it sounds like an small fan. i cant seem to figure out what triggers it to come on or what turns it off. it seems to come on when i first start the car but not every time. Its loud and very noticeable.
Any ideas on what it is?
How do i fix this?

 

Its probably the SAP (secondary air pump) going bad. What the pump does is inject air into the exhaust upon cold start to heat up the cat converters faster, it lasts for about 1 minute then turns off. When they go bad it could either sound like a vacuum cleaner or a loud whine. If you can locate the SAP pump under the car disconnect the electrical connector and wait for another day to try a cold start. The pump is a large plastic pump that has a hose that goes to the air box, follow the hose from the air box down to the pump if you don't know where it is.