ML 55 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 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. 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...