I have a '07 A4 2.0T Quattro with 34,000 miles on it. When I start my car after it sits overnight, for the first few minutes the car seems to run rough- and it "surges" a bit. You can feel it and see it on the tach (a variance of about 250 rpm). This may/may not be related, but I'm also geting some odd exhaust sounds- when it's cold, I get a "flap" sound, much like you'd expect from a Honda Civic hatchback with a fart pipe on it. If I accelerate at any significant rate, during this time you can also see significant soot clouds from the exhaust. After the car is warmed up (~5 minutes), everything is fine.

I've taken it to the dealer 4 times now, and they cant figure it out. There are no stored codes or CEL's to indicate what the problem is. Obviously my factory warranty is about to expire, so I'm looking for any advice to pass onto my mechanic.