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.