Berny,
The old Hobie rudders used to do that until you shaped the trailing edge of the rudders (daggers, too, in your case). It is my understanding that the blunt edges made the water cavitate and therefore created the humming. Sounds pretty cool and you know you are going fast, but it actually slows you down. I just got a new dagger for my A-cat and either it or the rudders were humming like crazy last weekend. I'm going to sand down the edges this weekend and see what happens.
I helped W.F. put some F18HTs together last year and he had us shaping rudders and daggers. We started wet-sanding (using blocks) with about 400 grit to knock off the edges, then came in with 600 to make it smoother and increased the angle, then we hit it with 800 and finally 1000+ to give it a nice smooth edge. We came back about a half inch from the edge of the blade. When we were done, they were razor sharp. Remember to sand in the direction that the water would go over the foil! On the leading edge, we really just knocked off the mold line to give it a smooth edge.