What I had just read was about Hobie, so no label bashing intended and my sentiments go for all manufacturing names in all areas of life. Unless you are in the local cheap to buy outfit getting a Chinese made Pingwoo socket set for thirty dollars.
Because that is where the production values are pitched if they are not excellent and ready to go on a brand new item.
It is clear you are better off buying a near new, just raced boat that has been sorted by grown ups.

Sometimes there is a low spot that runs from top to bottom along the trailing edge. Filling this "trough" may stop the hum.

To define the area; rock a straight edge along the sides of the trailing edge. You should be able to see light under the straight edge if the rudder is not fair.

Lacking that, spray some lacquer or other fast drying paint on the sides near the trailing edge. You want a very light coat, so just dust the area. Then, using a fine grit wet/dry sand paper and hard rubber block, start sanding the lacquer. After a few strokes the unfair area will have retained the lacquer.

Fill with thickened epoxy and sand fair.

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)

Call this guy:
http://www.recreationalcomposites.com/

Genius. Did the rudders on my NACRA 6.0 Express, no hum, finger tip control,sweet. And I don't even know what the helm on a cat should be!

Seriously though, he did the foils on a trimaran I sail on and the performance difference was dynamic.

yep