Why is my iPhone playing the notification sound but no notification is showing? A few minutes ago I was listening to music on my iPhone and the music faded out and then the notification sound played but no notifcation showed. (it has done this about 10-15 times now) Does anyone know what could possibly be wrong with my iPhone? I have an iPhone 3GS running on iOS 5.0.1.

Sometimes it's an email sound but other times it's the actual notification sound. I have it set up to show the little notification at the top for new emails so it should show a notifcation even when the email sound plays. Most of the time it shows a notification but sometimes it doesn't. 

Maybe this is just a bug?
Check if you have the new Gmail app installed.  Every time it gets a new email, it triggers the "iphone Text Message sound".  The sound can be disabled via Settings app > Notifications > Gmail > uncheck Sound.