Yes, I realize that a yacht can go faster then the wind speed, when close hauled, but I find it difficult to believe a wind turbine just keeps accelerating to infinity.
Infinity? No. Speed of sound? Yes indeed.
A 3m (10 ft) set of blades will accelerate up to 2000 RPM if there is nothing else to resist the rotation but drag.
That is exactly what Paul experienced, including the noises (aircraft and helicopter propellor tip speeds are up to Mach 0.9).
I made the same mistake with the first propellor I built. (Wow 20 years ago...) I put it on a fencepost to spin free and left it unattended. It was a calm afternoon, and wind rose up later. Came back after supper and picked up bent and broken pieces.
You always need a load on a turbine rotor, preferably with some redundancy.