Hi Gilley-

I have been browsing these forums for a while, you seem to be the guru of tech. advice & I need some help. My 2000 ML55 has had a rapping noise at cold startup since 20K miles (45K now). Mercedes has replaced rocker arms, lifters (twice), and about 1 year ago diagnosed the problem as carbon buildup. They did a carbon cleaning which took care of the issue for about 5 months and it came back. They then had mercedes regional tech look at it and the carbon was back. They told me that it was the gas I was using. I only use 93 octane gas, mostly shell V power. My commute is 35 miles each way all highway at 80mph, so short of jet fuel Im not sure what else I can put in the car. They gave me Techron additive and told me to use it every 4th tank or so, which I have. The rapping is now back again. I'm not sure what else to do about it.

Have you ever heard of a carbon buildup issue like this? Any suggestions you (or anyone) have would be greatly appreciated, as to how I can get rid of this issue, I am at my wits end with it. Thanks.