Do you mean the mechanical vibration of the coil? When the current flows in the turns of the coil, they experience a mechanical force and this may give rise to a mechanical vibration of the coil. This will change the physical dimension of the coil and may lead to undesirable effects. This frequency will depend on the spring constant of the coil. Many coils are securely mounted on a former and are relatively stable or rigid.
3D conical coils without support are relatively more vulnerable.