Nailing = tapping (not slamming) two hammer heads together in time with the engine.

Normal 'clacking' = hitting the wooden handle of one of the hammers with the head of the other.

Quick test - open the hood and from a warm idle, slowly bring up the idle by hand. Do you hear an increase in a higher pitched almost metallic sound, either intermittent or exactly in time with every revolution? Does it go away at 1600-1900 rpm?

ALSO, 'internal' leaks are more likely if you consider both the needle and the bottom of the spacer disc to top of nozzle body are TWO places leaks can happen - vs - top of spacer disc to bottom of upper injector half is one place.

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1987 300D (230,000 mi on a #14 head-watching the temp gauge and keeping the ghost in the machine)
Raleigh NC - Home of deep fried sushi!