The Pacified Purifier Exoskeletons have a skill which drops a GIANT beam of lightning on a location and triggers the accompanying sound. Once the enemy is defeated, the sound sometimes persists LONG after the skill has ended and the boss is dead (especially on expedition bosses). I have even ran back to pick up loot many screens away and it is still there when passing the same area. It does stop once you leave the level (so entering town or the next floor).