I recently encountered a guy who was not happy with the performance of his A6/18T. So he removed the exhaust catalitic converter and installeda straight section at that point. Now the engine has a rattle. No loose retainer bolts or anything. It sounds like a bit of mettal oddly rolling around in a chamber of sorts. The car went to the Audi dealer (to the head mechanic) and no one had a clue as to what's causing the rattle. Remember...just the converter was removed. He initially tried to sell me this car, but the rattle could not be explained. The chief mechanic says "other than that...the car works fine". I'm sorry. But this is an AUDI A6 18T and that is just not acceptable. Just think of the emotional stress of having to make excuses for that annoyance. There could be long term damage.