No its definitely something inside the hub itself as it only happens when span fast rather than ridden fast, I know the sound loose spokes make.
I've had the cassette body + axle out and sprayed GT85 in and it didnt make the slightest bit of difference, my LBS doesn't have the king tool but a friends one does so I'm going to borrow that and have it apart properly, I'd knock the bearing out if it wasnt already slightly notchy.