| Almost anything when hit, strummed, plucked, flicked or whatever will start to vibrate. Even buildings, when hit by the wind will start to vibrate (if you're ever on a high enough building on a windy day, you'll notice that the building sways from side to side). But simple things like guitar strings have a very clear frequency where you pluck them.
The frequency with which an object naturally vibrates when hit with energy is called its natural frequency.
The body cavity even has a natural frequency of 3 to 7 hertz. Natural frequencies are also important in phenomena related to resonance. | 
