I had this happen as well, my GS6 updated last night and immediately began a single, short vibration at approximately 1 minute intervals. It would even do this when on mute. With the volume on the phone sounded my standard text alert. I had no alerts (texts, emails, calls, etc...) showing but this persisted.
Closing all apps stopped the vibration temporarily but it started up again this morning. After trying various settings turning off notification reminders seems to have finally fixed this, or at least patched over it. I've tested this multiple times by turning reminders on and seeing if the vibration comes back and it does, every time. Just turning off vibrations within the notification reminders does not solve this problem, and in fact was already off.
Settings --- System --- Accessibility --- Notification Reminder --- Off