The more correct term is injector knock. Usually caused by incorrect fuel injection, so that the fuel ignites late and all at once rather than burning more or less smoothly during injection.

The term "nailing" has become popular lately -- a bad case of injector knock sounds very much like someone driving a barn nail into a 2" oak plank with a 16 oz hammer.

Unless it is caused by something like sidways injector spray or junk in the prechamber, it won't usually hurt the engine. Some clatter is normal in older engines, much less so in later ones.

Causes are low compression, late injection timing, bad nozzles (sticking open, bad spray pattern, sticking closed and opening late), plugged holes in the prechambers, broken ball pins, and burned prechambers. Leaking pressure valve seals in the IP will also cause a loud injector knock.

New injectors won't fix a knock from any other cause than a bad injector.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!