To dismiss the possibility of a fuel problem, run the engine at the rpm that induces tapping, loosen the fuel lines one at a time and see if cutting fuel to a particular cylinder makes the noise go away. If so, it's a dirty injector or some other fuel problem. If the noise persists, it could be a lifter. If you drive a lot, you might want to see if switching to Mobil 1 clears the noise. It won't happen overnight. It could take 10K miles for Mobil 1 to do it's magic and it's pretty much a life commitment at that point. If you're like me, 10K comes every 2 years so a quicker solution might be warranted. If you do replace the lifters, there is a specific loosening sequence for the cam bearings based on having the engine at #1 TDC.

Must be a very special 603 that revs to 6000rpm. The governor should hold it to the spec of 5000rpm +/- 200rpm

Sixto
95 S420
87 300SDL