Did you try seeing if the tick changes tone by pulling plug wires one at a time.... while running of course. If there is something going on with the valve train (or rod knock) I think it changes tone....? Maybe someone can confirm or deny....looking for quick n dirty diagnostics.....
I think that's more for a rod knock, if you remove spark, it eases the pressure on the rod hammering, changing the volume of the knock. I am not sure what effect it would have on excessive valve lash, but its worth a try i suppose.
