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.