The nut on a guitar serves several purposes; it holds the strings at a required space from each string and also determines the height of the strings above the first fret and above the end of the fingerboard.
Nuts come in a variety of different shapes and sizes. Made of either bone, brass, composite, or plastic, they are sometimes inlayed into the headstock just above the fingerboard end (Gibson or Martin style) or butted straight to the end of the fingerboard. They can also be inlayed into the fingerboard as in Fender electric style guitars and basses.
The nut is prone to wear and tear and should be checked on a regular basis. Due to constant string vibration, nuts can become worn and loose. The nut slots that house the strings should be slightly wider than the actual string for ease of tuning. If the slot is too tight for the string it will cause the string to take up tension in an uneven way and make tuning the string quite difficult. It the nut is too loose this will cause the string to move around inside the slot and create problems when playing.
How to make a guitar top nut here
Creaking and Clicking
When tuning, do you hear a repetitive creaking or clicking sound? If a slot in the guitar’s nut is too small for the string gauge being used, the string will bind and catch in the slot. A slight adjustment to the width of the slot is necessary.