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The basics of combustion are that the intake tract flows into the motor and the exhaust flows out. When a popping sound is heard coming up and into the intake system, there is a problem somewhere in the cam or valvetrain.
A failing camshaft can cause this. If an exhaust lobe is going flat, the only other path for the exhaust to escape is back up through the intake valve. This will result in a popping sound.
A burned intake valve or seat can also cause this. Once again, it is the exhaust gases finding another path to escape.
A compression test will identify the offending cylinder or cylinders. Pulling the valve covers and run the engine. Look for any rocker arm that appears to move less than the others. If the pushrod at that rocker is loose, you have either a defective lifter, a failed cam lobe or both.
With all of the problems lately with the low Zinc levels in todays oils, Camshaft failures are much more common.

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Exhaust lobe wore off cam.