I think what you are describing is fairly normal. When you turn off the ignition switch, it should cut off the fuel pumps and the ignition should still be firing as the engine revs drop. I seem to recall the ignition stays on for about two seconds. Both of mine pop a little bit as they wind down.
If yours pops a second or two after the engine quits spinning, I can't help you as to why but it would seem that any popping is a result of unburned fuel.