My surfski was humming like a thousand bees were on my back. A friend paddling ten meters away could hear it. I could feel vibrations in the hull...
Since I live far far away from Mr Fenn, I didn't wanted to ask for another rudder and have to wait for a few weeks to get it... Instead I re-shaped the rudder that was buckled and got it straight. Since then, no more humming nor vibrations.
The link below shows how it's been done. Sorry, the text is in french but there are some pics.
http://tuaruavaa.blogspot.com/2008/01/refaonnage-dun-safran-de-mako6.html