Ml55 Cold Startup Issue - Rapping Noise
 

Can anyone please help? I have a 2000 ML55 and have been complaining about a rapping noise at cold start-up for over a year. Mercedes has done a carbon cleaning which took care of the problem, but it is now back louder than ever. They did another cleaning 2 weeks ago because the carbon was back and claimed that there is nothing technically wrong with the car, and advised that I use better gas. I run only super and do not buy the cheap no-name stuff. One week later, the mass air flow sensor went, which they replaced. Drove the car for one day with the check engine light on, which they told me was ok. Now, the rapping noise is back and louder than ever - I'm at my wits end with this issue - can anyone offer any suggestions?

This car has 35K miles on it, 75% are highway...