Pop pop on exhaust side

The exhaust gases are an extinguisher, there should be no unburned fuel nor oxygen in there.
When the exhaust leaves the valve, it exits into high heat hell.
Hence the question how can it pop without oxygen, it must be sucking in somewhere.
I often get someone to plug the tailpipes with a lotta shop towels, cuz it's gonna get hot, then fire it up and quickly, (remembering the friend with hot hands), walk around front of car listening for leaks.
That's just where I'd look first.
Good luck.