They say that if you whistle at night, the spirits will think that you're calling for them.
That is what my friend said. But I laughed and said that she was superstitious. But one night, while I was relaxing in my back garden with a drink, I decided to see if my friend was telling the truth. So I began whistling. It wasn't a tune as such but I had fun. Then I felt a cold chill going down my spine and had an sudden feeling to go back inside.
I no longer whistle at night... but they do. And they haven't stopped since.