Sid:

The hydraulic tappets fill with oil when there is no pressure on them so that the cam follower actually rides on the cam at light pressure. When the cam lobe starts pushing down on the follower to open the valve, the small valve in the lifter closes off the oil vent hole, and the lifter becomes "solid" and the valve is pushed open.

When they get old, they no longer fit together as tightly as the concentric bits wear (notably the valve) and with hot oil, will squash down as the cam goes round. If the dont' pump back up by the time the cam comes round again, it will slap on the follower making the click you hear.

SEveral cures -- replace them (expensive), use synthetic oil in the hopes it will give you slightly higher oil pressure hot and dissolve some of the sludge preventing the lifters from filling, and (cheapest) check to see if there are some 0-rings on the center shaft of the oil filter cap (if you have one -- not familar with that engine). If there are two o-rings on there, no one ever replaces them, and when the get old and hard, the oi leaks out of the lifter gallery on some engines. New o-rings will go a long way towards fixing the problem.

Peter

__________________
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!