Audio Moiré Patterns

Woke at 3 am to the sound of banging. It was incorporated into my dreams until I woke up and realized it sounded like someone was sledge-hammering down a door in the neighborhood.

Then, wide awake I kept hearing sirens and when I would turn off the fan there was complete silence. This happens every summer, I hear what I call audio moiré patterns from my fan. Speed two seems to prevent the imagined sirens.