I don't think it's as serious as a worn cam lobe. It's probably just a hydraulic valve lifter (well it's really a valve pusher). Sometimes, for some strange reason, oil drains out of the lifter and it starts tapping. The tapping on your car probably wears out when you accelerate and get higher oil pressure. A friend of mine bought a 1994 33 "Feeling" with only 1000 km on it. He used synthetic oil from day one, and replaced it every 10000 km. It now has 50000 km and it has developed the lifter tapping noise. Only a small percentege of wear will make it tap, but it doesn't seem to affect the engine in any seroius manner.