Do you have your iphone switched to vibrate? If so, this alert could be many diffrernt things. The most obvious could be a new email or text message; however, it could also be a notification for some other app that is running on your phone. Try closing all the programs on your phone by double clicking the home button. When the multi-task bar at the bottom pops up, push and hold one of the apps. When you do this the app icons will start to wiggle and a red minus button will show up--push on the minus buttom for each app. This will close all the apps open on your phone.
If the vibration stops I would guess that one of your apps was causing the issue; if not, could be a malfuction of some kind.
Hope this helps,