Noisey
That's interesting, when the Westland Scout used to start up it made a similar sound, but is obviously only a single engine.... I always thought it was the point immediately before the engine thrust hit enough power to start rotating the blades, and was the deflection of the thrust?
It would not be the first time I am wrong and I am not a pilot and this is going back some years, but to me it was one of the hallmark noises of the Scout, nice to hear it on a modern bird though.