Popping in the exhaust is usually caused by unburned fuel entering the pipes and finding sufficient oxygen there to re-ignite.However this often causes multiple pops if it a carb issue. If its just one, its usually exhaust related. Like mentioned. a leaky header flange or even a collector gasket.
But since its happening under decel, a period of high vacuum, and when the cars momentum may be driving the engine against compression,and closed throttle, there shouldnt be enough fuel in there.Unless maybe its pulling it through a bad PV, or a too-far-closed transfer port.
I wouldda thought late timing, but 20 initial sounds really close.
IDK. Im kinda outta my element with 2 X 4s.