When you are fairing the back edge of the daggars or rudders, it would be wise not to make it perfectly square, or you may still get the humming (see the earlier post dealing with vortecies). The key is to make it slightly asymetric.

Think of the trailing edge of a wing; Maybe offset your foil's trailing edge by 5 degrees on that 1/8" of flat edge. Just enough so the vortex created is ever so slightly out of alignment and not able to resonate at any symmetrical period (causing the hum)