By default, the iPhone keyboard is set to make a clicking noise every time you press a key. Some people like it, but for others the sound of someone tapping away at a text message can be pretty annoying.
To turn it off, open Settings and select Sounds. Swipe down to the bottom of the menu and toggle the Keyboard Clicks option to the off position disable it. Now you can text away to your heart’s content without those pesky noises!