Vibrates For No Reason... WHY?!?!?!?!

My phone will sometimes vibrate in threes like (fwompfwompfwomp) really fast.. or twos like (fwompfwomp) for NO REASON. I wasn't moving into or out of service areas.. I was just sitting here. With full battery. Absolutely NO stimulus. I'm losing my mind trying to figure out why. SOMEBODY HELP!

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