Mystery Vibrate That Is NOT Email Notification??

Exactly how it sounds... My iPhone 4S running iOS 7.0.4, vibrates on occasion, for reasons I can't explain.

Before anyone throws the usual suggestions, I've spent half an hour or so Google searching and can clarify that it's not emails as I only use a gmail account which hasn't had an email since 13:00 this afternoon (last phantom vibration was about 10 minutes ago) and I have the app for that anyway. It's not the psychological thing because I can hear it vibrate on the table as can other people. It doesn't vibrate when changing from WiFi to 3G because it happens sometimes even when WiFi is actually switched off. I know I'm not flicking the "silent" switch on the side of the phone because it often happens when it's not on my person. I can also rule out vibrations on the hour or any other regular interval.

It doesn't happen every day, or at any set times. There appears to be absolutely no common factor in what makes it vibrate. There is never a notifcation, email, text message, or anything for that matter!

This might just be a waste of time as it sounds like a fault to me, but if anyone else is having this issue it'd be great to hear some solutions or somethin I may not have thought of?

Thanks!