I have has this exact problem on my c280 for 4 years now. I know it's nothing internal, as it bears none of the valve noise hallmarks and I have had my camchain/tensioner replaced with no effect. It happens cold only as the revs climb initially after the engine catching. It goes away almost immediately, but is embarrassing in a crowded parking lot. I'm guesing tensioner assembly myself, having had experience with the awful noise a bad one on my old 190e could create right off idle. If you rectify this one, please let me know how. I might try the replacement myself at Christmas if I get my holidays approved.