Ok, since you said you swapped the rockers from those cylinders to another area of the engine and the noise remained constant to the #5 & 6 cylinders, you might have a bad cam lobe or two on the cam on those cylinders. With the valve cover off of the engine, can you spin the engine over with the starter and see if the valves seem to be opening the same on 5 & 6, as other valves at the front of the engine. Take the coil wire off of course.
