How can I determine what is causing my iPhone to buzz?
Go to Settings->Notifications on your iPhone and then look in the section Not In Notification Center. If there are any apps listed there, you can see if notification is turned on or off for them. If any are turned on, you can turn them off one by one to see which one is trying to get your attention.

You may also be able to see badge numbers changing on one of your apps which would be a clue that there might be a notification too.