This is an odd situation that just started happening to me in the past few weeks. I'm not sure even how to explain it properly. Both my iPad2 and iPhone 4S (tied to the same account of course) at odd times will make a notification type chime but not the standard notification sound. If you look at the pad or phone, nothing pops up on the lock screen or home screen. I have no idea what it is or how to turn it off. Anyone have an idea? Thanks.