When one is tight, almost a certainty that another is loose.... the tapping noise is from the loose one.

Exhaust valves tend to go tight on the diesel from valve wear. Make sure the clearance doesn't change if you rotate the valve -- if it does, the seats and valve seat surface are worn unevenly and you need a valve job (and new rotocaps).

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!