Glow plugs do not have anything to do with knocks, or nailing, after a diesel is started up and running many minutes, or up to temperature.

If your diesel engine is knocking, and or what we call "nailing" if this is all in the same, less "rod knock", which I would very highly doubt in any Benz, it is (injectors nailing) In this case, you may have worn injector nozzles, or out of balance injectors, to begin! Lets not get too deep, and assume the easiest befor.....

#1. Install new fuel filter.....#2. Run direct (by-pass tank) Red-Line, LubriMoly, or Power Services, diesel fuel additive.....it'll run solely on this stuff, trust me, and it will clean your fuel delivery all the way to the combustion chamber and is safe......

Try this and then report back.......BB