This is a fairly common complaint. Could be from a couple of sources:

There may be excessive up and down play in the rocker arm assembly. Especially on the /5s and /6s where they don’t have any self-centering rocker arm mounting posts. With time, they can get loose and noisy. When you torque the heads, prior to setting the valves, loosen one set of nuts that secure a rocker arm to the mounting posts. Use a large set of channel lock (water pump pliers) or a C-clamp with two sockets and firmly but not REAL tightly squeeze the assembly together whilst tightening the two nuts. Torque those two nuts to the recommended torque. Repeat with the other side and torque the two nuts at 6 o’clock and 12 o’clock as well. Set valves on that side, go to the other side and repeat. Still have the noise?