The hum is caused by vortex shedding off the trailing edge of the blade. The vortices shed alternating from side to side across the trailing edge and resonating through the blade. It is usually worse when sailing downwind when the blades are lightly loaded and the pressure on both sides of the blade is the same. The fix is to bevel the trailing edge at an angle so it is not symetrical causing the vortices to shed from only one side. Shaping to a 'v' as with sissors suggested above seems to work also. Theoretically a square trailing edge would be the worst possible condition but I have had blades with a square edge that never made a peep, go figure.