The first crack was more of a pop. I actually flinched as I half expected coffee shrapnel to put my eye out. The second crack was more crackle than pop. Like the sound tinder makes as it catches fire. It was fun seeing the roast happen right there in front of me and it was much easier to hear what was going on without the fan motor whirring but I won't be abandoning my air popper any time soon.
I used a non-stick wok-like pan with a flat bottom. I agitated the pan and stirred them almost continuously but still, as I expected, the roast was very uneven.