At my first airline we were flying factory new Jetstream 31’s. Every airplane was being written up for its prop sync system which, when turned on in cruise, would cause a terrible harmonic. As a musician I started questioning what might be interfering with the sound waves between the two props when it dawned on me that we actually had a third prop on the airplane, a model airplane prop that blew air forward, from the overhead rear of the cabin. That small diameter two blade prop actually sent a sine wave forward that caused the two four bladed propellers to go out of sync! The simple fix was to change the diameter of the little prop.
Big problems sometimes only require small fixes.