The "howling/whistling" noise could very well be coming from the fin, and as you suggest it's almost an "audible knot meter" as it only occurs when you are really going very fast.
The most common reason this happens is that the trailing edge of your fin is "thinned out" a little too much, or sometimes there is a little "concave" area just ahead of the trailing edge that's too thin.
You can check for the "concaves" with a small hardened and ground dowel pin or a precision straight edge. Roll the pin or drag the straightedge down the foil of your fins perpendicular to the vertical axis.
If you see light coming between the pin/straightedge (now resting on the back half of the foil from the thicker part of the chord to the trailing edge) then you have some concaves and the best way to fix them is to get a flat sanding block and work down the fin to remove any "concavity" between the max. foil point and the trailing edge of the fin. This will "sharpen" the trailing edge, so when you have all the concaves worked out of your fin, you need to "blunt" the trailing edge slightly.
If you have no concaves, and the trailing edge is simply too thin and setting up a hydrodynamic "vibration" (the source of the noises) then you simply need to "blunt" the trailing edge.
To do this, take a sanding block and lay it perpendicular to the fore and aft axis through your fin's foil. With "smooth" (320 grit) abrasive cloth on your sanding block, run the block down the trailing edge of the fin 3 or 4 times. This will create a very tiny "flat" on the trailing edge that will be thick enough to eliminate the noise.
There will be very sharp edges at the sides of this tiny flat, so take your sanding block and put it at 45 deg. to the fore and aft axis of the fin and run it down the sharp edge once on each side to "chamfer" the very sharp edges slightly.
Try your fin on the water.
I'll bet the "howling/whisling/singing" will be eliminated. If it still sings, but at a lower pitch (your fin now "growls rather than howls"
) try the sanding block down the trailing edge a few more strokes and then the 45 deg. chamfers on the sides of the flat.
This should quiet your fin down so that you only have to learn to "dance" with your board and fin.
Don't think you could dance to the "howling fin serenade" anyway as the pitch is way to high. You'd shake yourself apart very quickly!
Hope this helps,