Since buying my S3 back in 2003 I always wondered what the rather loud whirring noise heard for a couple of minutes after cold startup was.
I recently found out it was a noisy Secondary Air Pump. When I changed my oil recently I noticed a broken plastic tube passing from this pump up to the top of the engine. I repaired it and the usual noise has since been accompanied by a sucking air sound indicating the pump was now doing its job. I never noticed this sucking noise before, so I assume the tube had been broken for the entire time I've had the car.
That was a couple of weeks ago. Since yesterday the noise of this pump has become alot louder and sounds very unhealthy, like its on its way out, so I started researching what its actually for.
I was surprised to only find one thread about it on these forums (strange noise on start up & turning off. I assume very few others get this noise, and possibly this pump isn't even fitted to post-facelift models?
Anyway, it seems that it only serves to heat the cat up a bit quicker at startup, reducing emissions. I'm guessing I don't need to worry about this too much as with the tube broken it obviously hasn't been doing anything useful in my car for years. Guess I could disconnect it to get rid of the noise rather than replace, although would probably get a fault code?
UK MOT doesn't check cold start emissions right? Although I heard in the US they are much stricter and most cars have these pumps.