Whenever a unit spawns, a scourge is killed over the unit. If you hear a scourge death, it should be only because your character was walking and the fog of war hadn't yet updated to your current screen position.
Pick a character, then use the civ to toggle spawning of enemies, down is off. Move your character to the green vespene labeled as PvM (player vs monster).
Map File Here
It works by giving the burrowed units currently on the player's screen to another player. Then, it counts the remaining burrowed units, and picks a random one leftover to spawn an enemy at, and orders the enemy to you.
None.