I used to race catamaran sailboats and the rudders that came from the factory would break into a high frequency vibration at high speeds. This would make a sound similar to the humming of power lines in a strong wind. The cure for this was to file the trailing edge of these rudders to a squared off taper instead of the rounded trailing edge that they came with. You couldn't file them to a point because you would file through the fiberglass skin and expose the foam core.
Theoretically, a razor sharp wedge shaped trailing edge is ideal but a truncated wedge is nearly as good while being far more practical.