Why does the iPhone 13 Pro Max have an electric shock sound on the back of the device?

Only Apple Store can fix it. You should reach out to Apple Support directly. Depending on your region, you may be able to reach them here: contact Apple Support 

or 

click on

 your device probably needs service.

Just thinking about what you described

Does this stop it?

1. Settings. Sounds Haptics, System Haptics, Off
2. Settings, Accessibility, Touch, Vibration, off

If they stop the sound then what you hear is normal vibration .

You're welcome, good luck.