Thanks for replying,
@ 101Sean and Aragorn, I will check the manifold gasket for leaking. When I fitted the manifolds I used a brandnew gasket but probably it doesn't make the exhaust manifold leak-free.
Is there a methode to check if the gasket isn't right?
@ Simon Smith, During assembly off the head and head-parts I checked everything on wear. The rockers had no or very little axial freeplay en where not worn at all. Between rockerarm and valve stem there are little valve stem caps. None off them where worn and I re-used them. But I will check them as well, cause I don't want the ticking noise anymore.