You have a bad glow plug or two. Test and replace, or replace the set. The light coming on after start is a diagnostic circuit, telling you that at least one is out.

Note that a bad belt tensioner shock sounds just like a bad valve slapping at idle -- apply pressure to the shock and see if the noise stops before getting excited about bad lifters.

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!