A self-modulation mode of nonlinear acoustic vibrations of cracked defects has been analyzed and observed experimentally. Spectral patterns of the self-modulation are attributed to the sum-frequency type resonance in a multi-degree-of-freedom system and include a subharmonic spectrum as a particular case. Both the subharmonic and self-modulation modes demonstrate a high localization around the defect and provide opportunities for early detection and recognition of damaged areas.