I used to sail a Sol Cat that got to humming at around 15 mph. It has to be the wires... nothing else can set up such a fine vibration that it is heard and not felt. Rudders would certainly be felt in the tiller. Rudder flutter would also be subject to specific loading... so if the whistle is at same speed be it what ever reach... rudder loading would be different in different reaches.


Additionally... vibrating could be the air stream uniting at the back of the stays. Like a flute.