Paul,

That is a common problem. The trailing edge of your rudder needs to be filed. When fabricating the rudder there is a minute "bulb" that is sometimes formed along the trailing edge of the rudder. When in the water and at the proper speed the rudder vibrates and sets up a harmonic wave which is the "hum" that you here.

On the Hobie web site look under "help with repairs" and "frequently asked questions" there are numerous posts on this subject.

When you do work on your rudders be EXTREEMLY careful with your EPO's. There is not a lot of material to work with.

John