Bad chain tensioner is allowing the chain to slap on the guides. It can get so bad with a worn chain that the chain will "fly" off the cam sproket and tap on the valve cover (at which point it usually also jumps).

Check the wear (stretch) on the chain (do a search!) and if worn, replace chain, tensioner, and the tensioner guide if it has slots worn in it.

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!