I hate to join late, but I have another solution to this problem, and no it's not oil. I tried that too.

It's the head gasket. When I changed mine, the tapping went away. On close inspection of the head gasket, I saw that some oil was leaking past the gasket around the oil gallery that feeds lifters for number 5 cylinder. This reduced oil pressure to those lifters and caused the intermittent tapping, especially before fully warm.

But I have an epilog that's not so rosy. 7 months later, that damn tapping came back a few days ago. Now it's even when fully warm, only at lower RPMs. So I have to consider replacing lifters or checking the head gasket again. I'm not certain that my dealer checked the head for trueness when they did the first gasket job.

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Thx/Reno
An American in France
88 300TD 5spd Sportline (lowered factory susp) - totalled by a flying deer
89 Range Rover V8 - the 4wd beast
02 Toyota minivan
05 Peugeot Partner 4x4 Dangel
88 International Harvestor 633SA
--Gone but not forgotten:
1970 250/8 C (sold to buy 450 SLC)
1972 450 SLC (sold after battling wiring harness problems too long)
1971 300 SEL 6.3 (sold after destroying two *very* expensive rear LS diffs)
1986 560 SEL (now my little brother's pride and joy)