Thanks propchief

I pulled the cables off the control horns, checked the cable motion, checked the rudder travel, etc, and saw no obvious problems, but when I reassembled, the singing is very much toned down - just a normal digital centering sing, not a buzz. The only thing I noticed was that the cables had a bit of torsion, probably from adjusting the cable length with the adjustment screws. Perhaps that did it. Although I did not specifically find the initial problem, it seems fine now. Thanks all.