What app is making my phone vibrate, without putting a corresponding notification up?

There is an app I can't locate that is constantly making my phone vibrate. When it does, I immediately check the Notification Center. Nothing new there. The vibrate could be a setting for an email application, or some other app. One thing to note, I always have my app on silent/vibrate. I just unmuted it in the hopes that the sound will help me figure out what is going on. **Question** How can I determine which app is sending rogue vibrations without any notification?