My car was making this popping sound after regular intervals when my car was either idling of moving.
Turned out it was because of a missing screw on engine. Air was going directly into the engine and it would make this popping noise. The car was also not idling properly, once the whole was closed with the screw, the idling became perfect.
There other problem that I had because of the same problem, when it rain, my car would not start easily at all. Even if it did it would die immediately. Taking the car a mile would be a pain as it would constantly loose power when I would accelerate and would stop. This was cause by the same issue, because air going in the engine directly. The engine would suck in air but since the engine would be wet, it should suck some water (this is my mechanic guess and mine), which would make the problem worse.
Hope this helps some body.