zz - the original poster implied/stated dinghies (usually with thin flat plate boards) such as 420s and lasers.
Nontheless, EVERY solid is by definition elastic to some degree and will vibrate when induced at its natural frequency or a harmonic of the natural frequency of the object. The 'flex' (amplitude of vibration) doesnt have to be all that great as the 'heard sound' may be further amplified by the adjacent shapes/geometries .... like the 'sound box' around which a guitar, etc. is built. It can also happen because the turbulance @ the edge/tip is in harmonic sync with the natural frequency of the object thats vibrating ... many many causes for a solid object to 'vibrate'/resonate, I only listed the principal and most common causes.
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. Or am I looking at this completely wrong? Not trying to be a smart ass, just trying to learn.