It was a common problem on 99 MLs, the fix was to change out the lock. This of course was under warranty and they changed them all.
It had to do with a spring, the motor tried to lock the car but the spring pushed it back up, that is where the ratcheting sound came from.
There must have been a bulletin, to the dealers. If they dont do it willingly see if you can arrange a meeting with the local MB rep.
It is not that easy, some pop rivets and the regulator have to be replaced, My ML ws the worst MB product ever, I traded it back after a year.