A ticking that occurs after the engine has warmed up COULD imply that it's related to the changing viscosity of the oil as the engine heats.

It could also be clicking due to increasing clearances as the engine heats up.

As said before, it could be a collapsed lifter. But it could also be an intake manifold gasket leak or an exhaust manifold gasket leak.

Have you ever tried removing either of those parts, or have had those parts ever been replaced?