I bet it's not this but simple to check...
I had a really annoying ticking for ages, just like tappet noise. After much ****ing about it turned out to be a slightly loose spark plug. I could just hear it sparking, which from where it was sounded like a ticky tappet. Tightened up and noise gone. This was after I'd replaced tappets, cam, timing and rocker gear.:doh:
Worth a check just in case. Good luck.