I have an HTC One M8 running Android KitKat 4.4.4, HTC Sense version 6.0. I've owned the phone since Dec. 30, 2014.
Not sure how, or why, but about 3 days after purchase, I started to have a problem with notifications, regardless of whether the phone was set to silent, sound, or vibrate. If I don't address a new notification the moment comes in, my phone proceeds to vibrate every 3 minutes until I can get to the notification (I teach all day, so I can't get to my phone). As you can imagine, this is quite frustrating.
I have gone through every setting on every app (installed or purchased) four times to ensure the vibration setting is shut off. I have done a factor reset, and put my backup on the phone. I do not have a purchased app giving me notification reminders. I have contacted HTC for device support, spent 4 hours with them, and they have suggested I send my phone in. Not a fan of that.
This seems really similar to a setting on iPhone which allows you to set when you want to be reminded of missed notifications. I just can't seem to find where to shut it off on this device.
TIA for any help/advice.