I am having a very similar occurrence. At a steady rpm, 2, 3, 4 K no noise. Light to medium acceleration I get a tapping. WOT I can't hear anything. I once had a 4 cyl engine with a rod knock. This is nothing like that. When I disconnect my vacuum advance the tapping volume is decreased. It sounds almost as if I have just one cylinder that is detonating under light/medium throttle. Perhaps one lifter that gets noisy only under light to medium throttle? At any rate I will be tearing it down to check bearings, rods, broken rings, lifters. etc.
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Frank M.
75 Firebird
68 Firebird 400 RAIII
66 Chevy II 461 Pontiac in AZ
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