My daughters 2004 has a rattle on start up when cold which I would bet a bunch is the timing chain tensioner. It is a very familiar sound which I lived with on my 300E. I guess I should have her mention it at the next service but judging from past experience, if the dealer tried to fix it they would break something else. The 300E did it when new and never got worse. It was sold after 130,000 trouble free miles.