What you are trying to do is knock off the radius
at the trailing edge, this is where the water tries to follow
the contour/radius of the trailing edge causing humming,
rather than flowing straight aft.

I have been successfull with pulling a sharp knife with
pressure along each side of the trailing edge knocking off
the radius, and chamfer it at 30-45 degrees until the
trailing edge becomes 1/16". Takes about 5-10 min per rudder.