You need to pin this down.
Try this get the motor fully warmed up and pull each plug wire off one at a time and see if you find a cylinder that when the spark is killed the popping is stopped or reduced greatly.
If this takes place then more then likely have the fit g order wrong, or a ignition issue like arcing over in the cap or across wires.
At worst you have intake valves the are leaking which can be found by a leak down tester.
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