If you hear an iPhone vibrate when it's charging, you may have an accessory problem. Faults in adapters or cables may make the iPhone vibrate constantly or at regular intervals whenever you plug it in. Alternatively, the iPhone may have a bug, which you can cure with a restart or reset. Water damage and notification settings can also be potential reasons you notice your phone buzzing when charging.
Check Notification Settings
If the iPhone randomly vibrates when it's charging, you may just be hearing notification alerts for new messages, emails, app updates or calendar reminders. Check if notifications have been making the iPhone buzz by pulling down from the top of the screen to open the Notification Center.
Check Charging Devices
Another reason your phone keeps vibrating when charging is that you may be using a faulty power adapter or USB cable. Although you don't have to use Apple accessories, not all third-party products work on iPhones. For the best results, use accessories that Apple certifies as "Made for iPhone."
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If your charging accessory isn't working correctly, it may have difficulty connecting the iPhone to the power source, making it vibrate each time it reconnects. Disconnect the iPhone from the cable. If it stops vibrating, the accessory is likely to be the source of the problem.
Check the charger or cable for external damage and make sure that connectors are clean and debris-free. Examine the charging port on the iPhone to see if it has any dust or debris in it. If it does, clean it out gently, making sure to pull dust out rather than push it in.
If you have an alternative power adapter or USB cable, swap it for the original and see if that charges the iPhone without making it vibrate. If the iPhone charges without buzzing, the original adapter or cable is faulty.
If you don't have a spare power adapter or USB cable, you can switch your charging method to see if that fixes the problem. If you've been charging by adapter, remove the cable and use it to charge the iPhone on a computer's USB port. If the adapter plug was the problem, the USB cable may still work. If you've instead been charging by USB, try the charger cable in a different port. Or, if you have an adapter, plug the cable in and try to charge the iPhone that way. This may work if there was a problem with the connection between the USB cable and a port.
Shut Down the iPhone
If switching the charging accessories doesn't solve the problem, or if the iPhone carries on vibrating after you disconnect it, you may have a system bug. Hold the Sleep/Wake button and then swipe the Slide to Power Off slider to shut down the iPhone. Hold the Sleep/Wake button again to restart.
If the iPhone is still vibrating, try a reset. Hold down both the Sleep/Wake and Home buttons, releasing them when you see the Apple logo. The iPhone will reset and restart. If shutting down doesn't stop the iPhone vibrating, or if you can't get it to shut down or reset, let the battery drain until it is completely empty. When it has no charge left and the screen shuts down, recharge it.
Check for Water Damage
Water damage may also make an iPhone vibrate, although in this case the device will constantly vibrate and not just when charging. If the iPhone has been in contact with water recently, you may be able to fix this kind of vibration problem by drying the phone out.