My XF started tapping on cold start-ups about a month and a half ago, and it would go away after a minute or so. I had the oil changed right after it started because it was about time for one, but it continued. The good news is today I started it up and it was pretty quiet for some reason. I left it sit for a few hours and started it up again and it was still pretty quiet. It has 14k miles on it right now. I don't know why it would just go away, but hey I'm happy lol. I think my car knew it was in trouble or something because I searched on Benz forums like non-stop the last couple days about noisy mercedes engines, and now the noise goes away.

Another thing... A few people had an engine that would have a ticky sound to it on acceleration, but be quiet at idle like mine. They discovered something about getting some bushing installed in one of the motor mounts and the noise would go away. That is something I'll have to keep searching.