If it's a 30V V-6 there's a very good possibility that it's an oil leak. Very likely from the valve cover. My '98 A4 did it as well as the '99 A6, both 2.8 30V V-6, same engine used in many B5 passat's. Did the timing belt, water pump, etc. service and cured most of the problem. Good Luck, There's lots of BTDT in several of the forums.