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When the trans is engaged, the engine is under a load, this is when exhaust flow becomes important, remove exhaust pipes from manifolds on each side, retest. If ok you have a restriction in exhaust, sometimes the catalytic converter will break apart and lodge in other parts of the exhaust system, this may be the noise you are hearing intrermittently. Let me know
check ignition timing with a timing light. Disconnect spout connector at distributor harness, should be 10 degrees Before top dead center.