As you know, this is an electronically controlled transmission. There is no dipstick in the dipstick tube to check the fluid with in the field. Has anyone added anything to the fluid? Ever has any leakage at all? Are you still under warranty? How many miles?
First step is probably to check the trans fluid level. For most folks this will mean a trip to the dealer. The trans fluid temp has to be monitored using the MB "SDS" computer, then the shop tool dipstick used to measure the level. Very critical to check only at the prescribed temp (80 C). Also the trans computer (ETC) stores fault codes and this should be checked.
If all is in order (you DO have warranty left, right?) then I would call for the trans pan be removed and checked for, ahem, tiny little pieces of the trans that wish they were home where they belong. Critical on this trans to also ONLY use the MB trans fluid for the 722.6 trans. If the transmission has not been tampered with, then possibly MB and the dealer can help you out, even if the car is "somewhat" out of warranty. But I'm sure you have lots of warranty left, right?