I have noticed that when I set my phone down, it will sometimes vibrate/buzz. I can hear it buzzing on my desk until I pick it up.
It's not a notification of any kind, it will continue to do so until I pick it up and set it back down again.
It feels like a haptic motor in the upper center portion of the display is causing this. I have never seen anything like this before. It does not do it every time I set the phone down, but it happens enough that it is both annoying and worrisome from a device health/functional longevity standpoint.
Has anyone experienced similar and found a resolution?