Popping noises can occur from a compressor stall in an engine. Which occurs when the engine is going through the air at too great an angle relative to the direction of its travel and isn't scooping air efficiently.
In this accident, this may have occured as the aircraft went out of control.
Thanks for the info--you sound like an aeronautical engineer or pilot. If the popping was due to the engine angle, under what circumastances would that have occured first?