What is it? Driving me crazy!
You are probably either getting messages or notifications of some sort. Notifications can be moves you are playing in games.
If you don't like the whistle, I think you can change it to something else.
A two second GOOGLE SEARCH: The default Samsung sound for incoming texts is a bird whistle
Thanks so much, I went to Samsung and figured it out. Notification sound and I turned it to silent. I was going crazy when it started as I didn't know where it was coming from and it would go off all night! No more birds for me!