I'm not good with words and explaining much detail but I did find an informative post that follows what I think your problem might be. Without citing the source the OP did approve on copy/paste, here it is.
Sounds like an intake valve issue to me. What I think is happening is the valve seat sealing surface area is not holding the detonation pressure of the fuel. The good thing is if this condition was totally lost it would not be popping like that.
This condition may be due to:
Sticking valve stem.
Carbon/debris on valve seat, damaged/cracked valve/seat.
Weak/ damaged spring.
Worn keeper or worn/damaged spring washer.
Lifter pumping up?
If you continue to operate the engine, for other than diagnostic purposes, you may damage the head, damage the valve, or the engine.
You can look for the valve by removing the covers and observing or measuring the heights of the intake valve stem/spring up as compared to other intakes or at TDC or the start of the compression stroke (both valves up).
OR
Find the cylinder and valve by loosening the fuel line, while the engine is running, until this condition stops. Verify the cylinder by tightening fuel line to create the noise again.
Remove the cover and inspect the valve's height, spring, cap and retainers.
Sometimes it was a worn or damaged spring, more than that was a split valve cap and worn keepers. If the valve stem is nice and free, no wear on stem or guide- reassemble with replaced parts, including seal. Place piston in up position to perform these tasks.
At times, when the valve was sticking and hard to operate, I have cured this problem by cleaning and lubing the valve stem and guide. You need to remove the valve seal to perform this task-with the piston up to prevent the valve from falling down.
Other times when there was debris on the valve seat, I would spin the valve stem SLOWLY, in a drill, applying GENTLE upward pressure after spraying cleaner and lube down the guide/stem in hope for it reaching the seating surfaces. Most times this worked when the owner caught this condition early, the other times required a valve job or new head.
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