You have some bad tappets. They are easy to replace on this engine, look into it. Not all that cheap, alas.
If the noise goes away when the engine warms up, you can live with it for a while. If it stops going away, you need to replace the tappets.
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!