What you describe is called 'nailing' because it sounds like hitting a nail with a hammer--and is what the fuel ignition is doing to the piston and rod bearings!

A proper injection/ignition process goes like this: When the piston is coming up and reaches the point when fuel injection starts (the injection pump has to be 'timed' to start the injection at the right point in the compression stroke) and the injection stroke starts, the fuel is sprayed into the prechamber and starts burning with only a very slight delay (ignition delay), and the flame contimnues throughout the injection stroke, creating the high temperatures and pressure that drives the engine.

It is caused when fuel is not properly atomized during the injection pulse--and it takes too long to start burning. When it does, there is so much fuel already in the cylinder that it goes 'bang' rather than a continuous burn throughout the injection pulse.

Partially clogged injector nozzles are the most common cause. Faulty injector pump discharge valves will also cause this as they 'leak down' between each injection stroke causelow injection pressure.

First place to start is remove all the injectors and have them cleaned and tested by a shop qualified to do it. (truck repair shops do this all the time.) Maybe your MB dealer has the equipment most do not and just sell exchange injectors. 90+% of the time the nozzles can be cleaned. The fuel passages in the nozzle are extremely small and precise, and even one of the 3-5 holes plugged will seriously disturb the fuel spray pattern. I have seen some that shoot out a solid stream rather than the 'fog' pattern necessary