I agree, everything does flex to some extent. However, I think that the finish on the foils itself and the trailing edges (non-laminar flow) will cause the hum before a "flex" or vibration hits it's natural harmonic frequency.
If the turbulence occurs at the trailing edge, it should not cause vibration or hum - unless the turbulence wash from the keel is effecting the rudder. I would think at the higher speeds, the fluid flow is separating (Transitioning from laminar to turbulent) slightly behind the widest part of the foil. This would then effect the trailing & thinnest section of the foil - and noise (hum) could be generated.
I think you are correct in saying the surface finish is a factor, but it helps promote the vibration not the reason for it. Because of the shear stress the hull, keel and/or rudder generates while moving through the water - any disruptions on the surface finish will cause the flow to transition - resulting in the eddies and pressure variations that cause the vibration.
The word "flex" might be misleading - think of it more like the continuous shifting from tension to compression.
I do believe the vibration can and does promote fatigue in components (how much - is debatable), but does it effect speed? When I think of vibrations as mechanical energy lost into sound waves - the thought is that it could slow a boat down. Another line of thinking is it reducing the effects of friction - like the old football game with the figures moving on the vibrating game board. Any feeling one way or the other? Not that I an suggesting adding vibration generators to sail faster. . .if that be the case.