Despite many studies of marine mammal vocalizations, there is no generally accepted theory of sound production by whales or dolphins, and only qualitative descriptions of their spectra. Of particular interest are the unusual sounds made by killer whales, which appear to arise from two independent sources and are called "biphonic." We have analyzed a number of these sounds from Northern Resident Whales and find the resulting spectrum is due to convolution of the spectra of two sources. Since convolution in the frequency domain translates to multiplication in the time domain, it means that the sound of one source is amplitude modulating the sound from the second source. This is a non-linear interaction rather than a simple linear superposition of two sources. We are attempting to find a physical model which would explain this behavior and have a Matlab program that simulates the original sounds.