In burning wood in our fire place, we get a whistle once in a while; and looking at the fire when it is happening it appears that a small sap or air pocket ruptures and a tiny flame under pressure is present. Lasts a couple of seconds followed by a snapping or popping sound. In the smoker, that may be what is occurring. Being in a confined space, the pop may seem louder IF that is what is occurring. There is also the belch which may be the same thing. Lots of theories. If the wood has indeed caught fire, I personally don't believe that there is sufficient oxygen to sustain the fire for very long. But it is sufficient to take the wood from slowly charring and smoldering to something that can reduce the wood to ash over 3 or 4 hours. I cannot believe that I can get smoke for that amount of time from one or two chunks if the wood is actually in flames. The belch might be the wood actually catching fire OR the generated gasses exploding and extinguishing the flames. No window so no way to check my theory or guess. And it is not important anyway.