Chris,
From recent personal experience (
) it could also be due to a leaking inlet manifold gasket. I had almost cured my "popping on the overrun" problem by replacing an exhaust manifold gasket. Then the engine developed a very loud screeching sound due to a vacuum leak. Replacing the inlet manifold gasket cured that one, but also finally eliminated the overrun popping/banging problem.
Hope this helps.