I disagree...the humming is caused by a well balanced trailing edge. There doesn't need to be any cavitation (and this word is often misused; see below). The same kind of 'humming' can happen to airplane control surfaces if they are shaped very evenly. The foil, if balanced so well, begins a very fast and short oscillation (hum). In the case of the airplane example this can be severe flutter causing damage. If you just taper, very slightly, one side of the trailing edge of the dagger or rudder it will be ever so slightly unbalanced as the water leaves the board and it will settle down.

Cavitation is caused by very high mechanical pressures of which a sailboat is not capable of producing. If you shock the water with a huge drop in pressure, water vapor bubbles are formed. Shortly after that, the water would encounter normal pressures again and the bubbles would collapse with a good deal of force. Cavitation is common on poorly designed water pumps (causing long term damage to the pump or plumbing) and on boat motor props...Often sailors look at their rudder during a turn and see bubbles in the water trailing the rudder and call it cavitation. However, this is nothing more than the water curling on the lee of the rudder and drawing in some regular 'ol air from the surface. This is still not good since it decreases rudder effectiveness and is usually a sign that you turned the rudder too hard too fast.

Cavitation: (from dictionary.com)
1- The sudden formation and collapse of low-pressure bubbles in liquids by means of mechanical forces, such as those resulting from rotation of a marine propeller.
2- The pitting of a solid surface.
3- Medicine. The formation of cavities in a body tissue or an organ, especially those formed in the lung as a result of tuberculosis.