I was having this issue as well but it seems to have stopped randomly after a little. What I did was touch a magnet to the bottom on back and hopefully set it back in place. Hasn't stopped in over 24 hours. Another thing I'd recommend is if you're bootloader unlocked and you're turning you're voltage down to stop cause that could be the cause of it since the motor isn't getting enough voltage
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Well I spoke too soon... Just stopped vibrating till I hit it. Sigh :/
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