Mercedes R Klasse Transmission Issue

That does sound concerning! A faulty torque converter, specifically the lock-up clutch, aligns with those symptoms. The grinding noise especially points towards internal damage. Slipping and rough shifts are typical when the converter isn't doing its job. Does the grinding happen in all gears, or only specific ones? Has the transmission fluid been checked recently for level and condition? Dark or burnt fluid could confirm the issue. Ignoring this could lead to much bigger automatic gearbox problems down the line.

Grinding is mostly noticeable in the lower gears. I haven't checked the transmission fluid myself. Is checking the fluid level hard? What should it look like if it's gone bad because of a converter fault?

Checking the fluid level isn't terribly difficult on some models, but accessing the dipstick (if there is one) can be tricky depending on your specific R Klasse configuration. Bad fluid related to a torque converter issue will often appear dark brown or even black, and will likely smell burnt, like old cooking oil. Given the symptoms and the grinding noise, I'd strongly advise against driving it much further. A defective converter can send debris throughout the transmission, causing even more damage. A transmission repair specialist should diagnose this properly, including checking error codes, fluid condition, and possibly performing a stall test. Get a quote, but be prepared for a bill around 2120€ if it requires a new converter.

Thanks for the advice. It sounds like a torque converter issue indeed. I took it to a workshop, and they confirmed the converter fault. Getting it repaired.