Flat tappet? Did you put new tappets in or did you use the old ones?

If a tappet back there has no oil in it, it will clack until it pumps up.

I'd pull the valve cover and make sure nothing is mislocated or floating around, then drive it a bit to see if the tappet revives. You can check them in place -- they must not be easy to compress. If you can push one in with a screwdriver, it will be noisy until it fills with oil, or you replace it if it's bad.

I'd also check to make sure you DON"T have an exhaust leak -- a misplaced or bent gasket can get overlooked, so that you have a leak at the head to exhaust manifold!

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!