Your entire guitar vibrates when you play it: the top most of all, but even the neck and headstock are moved by the vibrating strings’ energy. Any loose part on your guitar may buzz or rattle audibly, sometimes only when certain notes are played. Parts don’t have to be visibly loose to rattle, and it can be quite a job to chase down the causes of some of these little noises. Often a loose part can be located by rapping with your knuckle on the instrument in various places. If you knock on the body and the noise appears to come from the neck, for example, then there’s probably something loose on the neck or headstock. If the noise occurs only when you rap on the back, there’s probably a brace loose inside there.
When you’re trying to locate a rattle, the headstock is an obvious place to start because things are screwed onto it. Almost every part of the tuner has the potential to come loose and start rattling. The nuts that screw down from the top have thin washers under them to protect the face of the headstock. They are screwed tight when the guitar is first assembled but may become loose because the wood compresses a bit. After the guitar is a few months old, it’s a good idea to tighten these nuts.
Another place to check for rattles is the truss rod cover. Underneath the cover, the truss rod nut may also be loose, and it or the washer under it can make a lot of noise. Even if the angle of your neck is just right, it’s generally a good idea to tighten the truss rod nut just enough to keep it from coming unscrewed and allowing the washer to become loose. The truss rod itself can graze the inside wall of its slot inside the neck and sing out loudly when you play just the right note. If you change the adjustment of the rod just a hair, you might keep it from rattling.
Onboard electronics are increasingly common in acoustic guitars. Adding a pickup means adding parts that can vibrate and make noise, especially if they become loose with time. Considering how the wires hang around inside an acoustic guitar, it’s amazing that they don’t rattle and buzz more often. If wires do rattle, keeping them quiet is a simple matter of adding little stick-on retainers to hold them in different positions.