First thing is you need to take it back to who ever scanned the computer and have them PULL the code from the computer not just reset it. The gas cap can cause the light to come on but I doubt that was your problem. Have them rescan it and get the code. Also AutoZone will scan the computer for free. Now is the popping sound in the suspention or the engine back firing? If it is the engine backfiring I bet the code that is pulled will help solve that. If it is something in the suspention have all the bushings checked...upper control arm bushing,motor mounts, sway bar bushing..etc.