Paul....
I agree with others here. I've personally experienced this fault. Where it's more of a mechanical, gear/chain nasty noise, rather than a sudden rush of air/suction noise!
Just to add to what others have said here though, my car a 2001 S3 had this problem when i bought it. It didn't put me off, i really did think that it was a heatshield rattling somewhere inside the engine bay or something which was loose, metal on metal.
However, it turns out that the VVTI unit and timing chain system was at fault. Was advised by Audi not to drive it and to get it done there and then. I.e leave the car with them.
It does all depend on how bad this is on a particular car but it is NOT supposed to do this when the engine is working as it should, even on startup. All the information i'm stating here comes from Audi GMBH and a main dealer head technician.
The issue with this failure is the oil return pick up pipework, which has a filter at the bottom end (sump) this when using Long Life service intervals gets cloggy and blocked due to the rough consistency of the oil over a prolonged period of time.
When the timing chain tensioner has no oil pressure to mechanically work it fails to operate the tensioner, thus leaving a slackened chain. Mine was overly noisy and was aparent during gear changes also. Which was kinda scarey and that is what made me orignially think oh the exhaust is moving slightly and this was the heatshield rattle etc.
So £500 lighter in the pocket (Dealer agreed to pay 3/4 of money required) i ended up with a complete new VVTI unit, timing chain, tensioner, oil return pickup pipework, metal prop water pump, oil pump, cam belt, sensors and all the other consumables which were required.
I had a full service carried out, then had another one 3 months later to make sure the oil was nice. Due another one in a month or so. In all fairness the car was and still is running much much nicer. I chose a specialist in the end as i felt that he would carry out a better job and i'd not have monkeys spunking over the car. I'm particular where i take my car to be worked on and i am still very very happy with my choice.
In my opinion, if you have the money to get the car done now, do it. It's only going to get worse like mine did and you will eventually have to get it done. You could sell the car as is, but if you are like me you love you car and won't want to part with it.
Just an insight from someone who actually went through it start to finish! Another forum user by the name of NAK has recently done this. He even had two different engine types and used the VVTI unit and timing gear from another engine code to fix his problem.
It can be done cheaply but i wouldn't want my chain snapping or my car underperforming. Timing chain is a very very important integral part of engines which use this method.
Hope that helps!
G
