Check out the rocker arm that has the noise.Remove it from the head and inspect the rocker arm ball and the inside of the rocker arm for large amounts of wear or galling/burning.Some original factory rocker arms had a rocker ball that was made out of a aluminum/diecast like material.They were soft to help quiet the valvetrain area when the engine was new.Over time,high miles,or heat damage they would make noise.Note,you may have more than one rocker with a noise.Aftermarket rocker arms have a steel ball with a series of slots that provide additional lubrication to the critical rocker arm ball area.
Good luck hope you find the problem.
