At last, I have found a good video explaining how to suppress the vibration of fins and foils (aka the Von Karman effect), and giving the name of this technique : the Donaldson trailing edge, described in 1956 by ... Donaldson
Up to now, I had only seen the diagrams for symmetrical foils but this video goes into the details and show that you must sand the inner face of asymmetrical foil (flat one), not the curved one.
Thanks to Mario Legenstein from Levitaz for the video:
Thanks Colas. Good to know how to sand the trailing edges properly.
At the moment I'm happy with the whistle or should I say scream or high pitched whistle when doing a ton of speed. But this is usually only when taking the drop down head high or bigger waves. It eases off to nothing after levelling off & riding the swell. So it's not annoying to me & is a good indicator to know when I'm in the high speed band.
But I definitely would sand off the edges as described if my foil whistled whilst riding after the drop because I love that quiet serenity it's the best after making the take off.
It's a bit like getting fired out of a cannon. You get the big boom! And then it's just plain sailing after that.