Whistles in the Night

You cannot whistle or play any wind instrument after sundown, or you would end up summoning a ghost, because ghosts sound like whistles when they move swiftly through the air.

This is an interesting superstition because it plays upon the fear of  ghosts, yet it also plays on contagious magic—how making a sound summons something that sounds just like it. This could be a more magical/convincing way for parents to tell children to not disturb neighbors at night.