Tapping/knocking sound

[Hammerhead]

Pop the rocker cover.
Check the Jake rocker arm's detent strength on each cylinder. It is the center rocker arm on each group of three. They should "snap" into ace, if they just roll into place, they're worn.
The Jake rocker's detent spring get weak over time, allowing the off center weight of the rocker to start sagging. This causes the jake rocker check valves to start to oblong the holes on the rocker shafts as well as making the sharp taper of the check valves wear in a radioused shape decreasing the ability of the Jake rockers to stay in the neutral position and they start to be "bounced" off the exhaust valve pins and then clang off the camshaft on the other end.
If you are jaking under high boost load and touch the throttle to interrupt the Jake's and you hear the clattering louder than normal with your window down slightly, then you can pretty much count on this being your issue.
The repair is;
New rocker shafts
Inspect the rocker bushings for wear, and replace if necessary (not likely)
Replace all 6 Jake rocker arm check valves and springs
Reassemble, and adjust overhead
It's not a difficult job, but it requires some specific tools.