I recently took my 2012 550 S4 to a dealer to have some work done on it. I was having trouble getting it to shift. They said I had a clutch problem so they replaced the clutch and a belt. I also had them service it will it was there. When I got my gator back the shifting problem was fixed but it now backfires when it is decelerating. I only had the gator home for an hour and had drive in at most 30 minutes. I immediately called and had them come back and get it. Any thoughts on what may be wrong?