I just drove my friends 1985 300D Turbodiesel back to Charleston from Tennesse where he bought it.

When the engine is under load there was not valvetrain noise at all, but when I would lift the accelerator pedal very slowly in order to slow down just a little or very lightly feather back the accelerator pedal I could hear a faint clatter, for lack of a better word, maybe kind of a tapping. Oil pressure according to gauge remained pegged at 3 bar. Oil level was not below low mark on dipstick.

I'm not sure this is valvetrain noise but thought it might be. Could it be a sticking lifter?

In any case what is the most likely source and what action should I take? We both have a bunch of cars but I'm the one who works on them.