Phone vibrates once, quickly approximately every 3 minutes. It's not exactly 3 minutes, doesn't seem to be any notifications. I've tried shutting off all notifications and still does it.
Has done it since day one
Any ideas ?
I'd try a few things if you don't find the issue in Safe Mode. Go to Settings-Accessiblity-Advanced Settings-Notification Reminders and see if that is turned on to remind you of notifications every 3 or 5 minutes.
Then go to the Galaxy Store and install Good Lock if you don't have it. Once it's installed, click on the "Family" tab and get the app called Nice Catch. It will provide you details going forward on any application that triggers a ring, vibration, screen wake-up, etc. It was handy when I had an app that was creating some type of background notification but I never had anything showing in the notification panel for it.
Good luck, that would drive me nuts.