My iPhone 16 Plus keyboard is making sound without using it

Please follow this procedure for force resetting your iPhone ⇢

• Press and quickly release the volume up button.
• Press and quickly release the volume down button.
• Press and hold the side button until you see the Apple logo 

Please post back if needed. 

What troubleshooting have you tried? I would first try a Force Restart on the iPhone. That will not affect any of your data. See if that clears the issue. Force restart iPhone - Apple Support

What are you doing when you hear this sound? Are you saying that you are hearing typing sounds when you are not doing anything on the iPhone?