I own a 2003 audi a6 3.0 quattro that I bought back in september 2010. It has roughly 150,000 miles on it. When I bought it the dealer had the catalytic converters replaced on it because the check engine light was on when I bought the car. Since then I have had the oil changed (full synthetic), transmission flushed, and the timing belt service done. Also since I have bought it there has been white smoke coming out of the exhaust. We cannot tell what this is coming from. The temperature levels on the engine are fine, oil temperatures stay around 220-250, and all my fluid levels have been checked by a mechanic and they are all fine. One thing that was noticed, although it hasn't affected it at all was when the timing belt service was done the mechanic took off one of the head gaskets and said there was some actual water in there (it did not look like antifreeze or any other fluids), he emptied it out and put it back together. Any reason this smoke is coming out or what I should have done about it? I also have recently used STP fuel cleaner twice when I filled up with premium fuel, and no difference.