Have Mercedes Questions? Ask a Mechanic for Answers ASAP
You say 1 knock, or continuous knocking until you turned the engine off?
Could be one of few things, but any loud knock from the engine is not going to be easy or cheap to repair.
How many miles has the engine done?
Ok, that is hopefully not too bad.
You need to get the car scanned for DTCs set.
The trans is in limp home mode as well, result of the low power engine problem.
Can you get the car to a good workshop or your dealer?
Have you had a chance for a workshop to look at your car?