Same Wokka in the West?
You describe exactly what I heard on the golf course 27/08/08 when a westbound wokka flew over Saltash above the low clag. The swish was of such a frequency it seemed to be from one blade only and it was only audible as the aircraft was approaching. As soon as it passed overhead the swishing sound could no longer be heard.
Think it returned from Culdrose to Odiham later that day as Vortex 424 sounding like all its brothers and sisters, but wasn't directly (dreckly?) overhead.