I've been chasing a strange noise in my engine lately.  Maybe someone has run into this before?  The noise is very intermittent and only happens when I rev the engine slightly. It is a kind of popping sound similar to a cracked exhaust manifold and it seems to be coming from the intake manifold.  The engine runs perfectly with no miss or hesitation.  I installed metal exhaust manifold gaskets and I have no exhaust leaks.  I adjusted and checked the valves and they are perfect.  I have a 1957 "B" manifold on my engine with a Holley 465 and an electric choke.  I have a PCV.  The center exhaust ports that lead to the intake are open. I suspected the noise was coming from the hole in the intake manifold where the original choke was connected.  I blocked it off yesterday and it seems to be better.  I understand there can be problems with the exhaust crossover in these manifolds?  What's the story?