In Showcase: Infantry near the end of the mission when I begin walking up the hill to go to the two possible locations that the enemy spotters can spawn in I occasionally hear a loud, banging sound from the north in approximately the same direction that enemies sometimes retreat in.
I don't know what it is and it seems to me that enemies de-spawn after running to the other side of the hill but if I'm wrong about that then its possibly something the AI is doing off in the distance that you can hear so loudly because sounds currently are heard loud even though they are far away. If the enemies are scripted to disappear after retreating then the sound must be made by something else.
Basically it sounds like some sort of collision or explosion. It's not anyone on my team shooting grenades though and I've never seen an explosion.