For the last few days my iPhone 5 has randomly started making a whistle sound, like a referee whistle, and does a short vibrate with it. I can't for the life of me figure out what it's supposed to be. I've checked all my notifications, and even went through all the notification sounds available, and it's nothing I can find. I checked all the different apps in the notification center settings, and tried searching, but can't for the life of me figure out what's causing this brief whistle sound and vibration.