Don't take all the play out of the tiller/tiller cross bar connection. If you do, you will not be able to tack. (I did it once).Swing the rudders side to side and watch the connection get tight and lose. The NACRA is not a real 3 axis connection and depends on play to make up the difference. About a 1/16" of play is needed.

It is normally the trailing edge that causes hum. Make sure everything is smooth and square.