My C240 exhibited this problem within 3 months and 5,000 miles of new. They replaced the entire resonance system and the problem went away. I do realize that there are always many things that can cause the same problem, but it seemed to be routine to them.
Additionally, the comment about the small engine. This engine is small in displacement, but amazingly powerful with a very broad power band for such a small engine. Mine makes power that many larger engines could only dream about. There is technology in this engine that was only found in Formula One cars not long ago. As far as I'm concerned this engine is an engineering masterpiece. It also has been out there long enough to prove itself not to have any of the achilles heels of many other engines, including the later model inline six engines such as the 103 and 104.
Stick to your guns and whatever the problem, I'm sure your dealer will get it fixed.