I'll bet the sound is a dirty injector. I first heard it 20 yrs ago with a new 300D, I was horrified! I went to the dealer thinking I had a mechanical failure in the new car I couldn't afford.

Service manager buddy changed fuel filters and added a bottle of Red Line 500. He then told me I had just witnessed a $99.00 injector service.

It cleared up immediately and bad fuel was the culprit.