You have the best of all possible situations... So be happy ! It is the noises that will only happen when traveling which are difficult to find.. Many uppity MB mechanics refuse to ride in the trunk or under the hood at even reasonable speeds.... even in parking lots....to find moving specific noises.... those snobs....

If it will make the noise when in PARK then you get someone to hit the throttle while you prob underneath the car... Be SURE they understand the concept of LEAVING THE CAR IN PARK......you might even apply the parking brake ....

Take a long metal tube with you and place one end to the little flap on your ear... and push the flap in to close the usual ear opening... do not stick the metal rod into your ear....

Then you can put it various places on the frame drivetrain or exhaust and have a good chance of finding your noise producer.

If it will do it in park it pretty much eliminates all the moving objects behind the transmission. It does not eliminate things in the trans... but that is not a likely suspect from what you have described... ( Why is there not an icon for "good luck" ? perhaps a four leaf clover ? ) Greg