At my dealer just last week, the tech that keeps my C320 humming was working on a C32 with this very problem. Howling and groaning noises from the torque converter when in the 3-4-5 lockup gears. The fault in that car was a fluid transfer in the trans cooler in the radiator and the contaminant is glycol in the transmission fluid which is fatal for the transmission. He said the problem was the Italian made radiators (can't recall the well known brand MB uses) and the car in question was getting a new transmission and a radiator. Warranty in this case and they have done more than a few.

MB uses a test kit for the trans fluid to determine the glycol level.

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Roger E.