Crapaud has it.
The hum comes from the flex due to the viscous drag forces causing the thin plate centerboard to flex. If these forces excite the natural frequency of vibration (or a harmonic) of the keel/centerboard, then the board will vibrate quite excitedly.
I have a hard time imagining keels 'flexing'. Turbulant flow around or off the trailing edge of a foil yes. But having thousands of pounds of lead or cast iron 'flex', even under 15+ knots, I just don't see that happening.