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My car used to do this. The metallic popping sound is the pop-off valve, it just does that. The rubber bumpers may be worn inside, allowing the piston in the valve to bang into the metal end caps - but only go there if your sound is really a sharp metal on metal "smack" when the valve releases.
My problem was a bad Aux Air Valve. It stayed open all the time, so to compensate I had to adjust the idle screw very low. When I let off the gas, the idle would transiently drop to 200RPM (the "true" idle), then recover (POP) and (vrooooom) back to 1500 and repeat. Especially embarrasing when I restarted the car at a gas station.
I've been assuming it's something to do with the length of the Aux Air Valve hoses, and a stable limit cycle oscillation? I replaced the Aux Air Valve, got the electrical hookup squared away, readjusted the idle screw and then the mixture, and now it works well.
On my '77 the idle screw has a LOT more than 2 turns of adjustment range - don't know if it's a different pitch than on yours, or if you have more adjustment to play with...
Good luck!
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