You are hearing a little backpressure and some unburnt fuel.
Most people will call it "burble" now if its a loud POP it could be something else like a small exhaust leak somewhere throwing the system out of a perfect balance of flow/backpressure.
Kinda like a bum-de-dum-bum-bum is normal when you let off the gas and your in gear.
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Life is good with a 6-71 supercharged 355 in a 72 camaro
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