My guess (and that's all it is) is that it's the airflow around the blades -- it's a loud humming noise, present even on an unloaded (no turbine) VAWT. Much the

My guess (and that's all it is) is that it's the airflow around the blades -- it's a loud humming noise, present even on an unloaded (no turbine) VAWT. Much the same way that ships' propellers create noise: it is primarily generated by the blade causing a partial vacuum behind the trailing edge of the blade, and the collapse of that vacuum creates the noise.