Most likely injector knock, could also be a bad chain tensioner.
While idleing, loosen the cap nut on the injectors one at a time. If the knock goes away when you loosen one, chances are it's a bad injector. If it doesn't, suspect something else.
Sadly, new injectors don't always fix knocking, and if fact can make it worse if something else is the cause.
Low compression from tight valves will also cause knock, so will late injection timing.
Check the timting chain for stretch (do a search) and adjust the valves, too.
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!