Nonlinear ultrasonic modulation, a promising fatigue-crack-detection technique, is investigated with various frequency combinations, and an algorithm that can comprehensively analyze the results by comparison with ambient noise is proposed. The magnitudes at the 1st sideband frequencies of the Fourier-transformed result and surrounding noises are measured. Experiments with aluminum specimens before and after fatigue-crack generation show that cracks are detected correctly in both pristine and damaged specimens. When the developed crack-detection method is employed while applying an external force, specimen defects are correctly detected, although the magnitudes of modulated waves decreased. The time and effort required for monitoring the structure's health can be reduced by using ambient noise as a reference.