Well it seems that 'sometimes' was the operative term that I overlooked in your case. Intermittent noise can be frustrating to track down. In my case pulling the valve covers and inspecting the cam lobes quickly revealed the source of the noise. Scoring of the lobe and associated rocker increased valve lash to a point that noise was the result. Changing of the rocker without changing the cam is admittedly a temporary fix, but worked in my case. It's possible that a 'lifter' is dirty or binding and takes a while to remove the lash. Hopefully a more experienced mechanic can jump in and point you in the right direction.