This paper presents an experimental method for testing composite insulators based on vibration testing. The method used investigated the propagation, signal shape, and distortion of excited mechanical waves under the influence of defects. The aim of the method was to identify defects in the core of a composite insulator that cannot be economically detected by currently available diagnostic methods in field conditions. Therefore, this experiment aimed to distinguish between the mechanical waves’ characteristics of damaged and intact insulators using inexpensive tools. This article seeks to provide a basis for mechanical vibration diagnostics of composite insulators by demonstrating that damage to the core can result in a perceptible difference in the characteristics of mechanical waves when testing within the frequency range of audible sound.