Harmonic sounds (or approximately harmonic) are strong evidence that something evolved or was designed specifically for making sound. If you have an oscillator

Harmonic sounds (or approximately harmonic) are strong evidence that something evolved or was designed specifically for making sound. If you have an oscillator where the oscillation is constrained to one axis, e.g. an ideal string, reed, or column of air, it has only one degree of freedom. The state of the oscillator is fully described by its current position. It has no memory, so when driven in a steady state its behavior is identical every cycle. Even if its motion is non-linear, this will only cause anharmonic motion, not inharmonic, which is harmonic in the musical sense. The overtones must be integer multiples of the fundamental, because otherwise there would be different behavior on different cycles, which contradicts the single degree of freedom assumption. But most natural objects have multiple degrees of freedom. If you hit a rock, stick, bone, etc. its oscillation is not constrained to a single axis. You can have multiple different unrelated frequencies. It's unlikely that all the prominent frequencies will be integer multiples of the fundamental by chance. To get harmonic sounds from a oscillator with multiple degrees of freedom takes skilled shaping of that object, e.g. tuning a bell by carefully shaving off metal at the right places. Harmonic sounds are evidence of living creatures, and living creatures are potentially dangerous. Therefore it's not surprising that we're good at detecting them.