Gradually escalating unidentifiable whispering. Everyone starts asking if you can hear something, you all start agreeing you can hear whispering but not what it is, then the paradolia kicks in and people start hearing what they expect to hear. The constant gradual increase in volume. Increases the fear and certainty in what people are hearing, but also disrupts sleep and wakes anyone who was sleeping. Soon everyone is awake, sleep deprived, and panicked.