Auto trans problem help
 

I have a mercedes clk 320 with 5spd auto trans (722.6) car has drove 140000 km. Recently it started to make a groaning noise from the transmision at low rpm (under 2000 rpm). If I give it gas there is no noise but it tends no mishift in 3.4.5 gears in d mode not so much if I change manualy. There are no code faults I have used my scanner. I have changed 13 pin plug flushed box and changed oil and filter there did not apear to be any water in the oil I have changed the radiator to be on the safe side.

Any help would be greatly appreciated