You need to get the prechamber out (or at least get the bits of glowplug out) and verify that you do not have metal chunks bouncing around in the cylinder -- you will end up with a bent rod and piston damage.

A broken off glow plug will cause a severe injector knock, clearly localized in the pre-chamber area.

A loud knock below the head, definitely in the upper block, NOT the head and NOT the bottom, is a bad wrist pin bushing.

If you ran it any length of time without oil pressure, the main bearings would knock -- a very deep, definite clank, not a snap or clack noise. Probably can be rebuilt if that is the case, unless you spun a main. Main or crankpin bearing knock is definitely in the lower block, deep sound.

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!