HUMMING RUDDER

I own a Carbon Mako6 and experience a humming sound from about 12km/h and up.
Has anyone else experienced this? I have removed the rudder and sorted out all rough edges, so can't understand why still get this humming. The alignment seems as good as when I got the boat and seems no different to my glass boat which gets a similar humming vibration, but at much higher speeds, mainly on runs.
Any ideas what could be done to get rid of the sound.
Hate to think that the humming may attract some of those ou's in the grey suits.
Regards,
Steel

This isn't your rudder, its your venturi drain!

This is the classic surfski sound as you pick up speed on the runs. Its the sound that fires your adrenaline because it equals speed.

I hear it from 15km/h+, but I suppose this depends on your ski.