Don...the big six is known for its valve train noise. You can pop the valve cover and check the timing chain tension with a long blad screw driver. While the V/C is off you can set the valves. Try to see if the noise is local to a specific cylinder or overall. Many of us can have had these heads apart and can provide some insight but you have to provide a lot more information, in terms of what has been done to resolve the problem...if your valves were just adjusted...when it started to occur/was it more quiet before etc.
No real mysteries to the great big six...wonderful engine.
George