What are the threads like in the alloy head which secure the rocker shaft and have they been helicoiled or similar in the past?
When the threads in the alloy head go and/or the helicoil insert starts to fail you'll get a tapping noise. In my 300tdi it initially showed as a mystery tapping noise where I jumped through all the hoops you have. Resigning it to the "... eventually something will break so wait..." file eventually there was oil on the driveway from a leaking tappet cover. When I went to tighten up the three nuts which secure the tappet cover, #3 rotated freely. See my
thread for details of the fix.
If you've also had to tighten up your intake and exhaust it suggests that the head has been cooked or has gone soft, so a hardness test might be warranted.