Transition Effekt.
This occurs when you come out of forward flight slowly into Hover and from Hover into forward flight aswell. And is absolutely normal but a more critical Flight phase, because on some Helicopters this Transition can be relative short and the Pilot needs a bunch of attention especialy on the Collective and the pedals.
The Wikipedia link Razor linked to you is more about Noise reduction overall to make Helicopters inflight less Noisy…The EC-155 is a good example on how Noisy Helicopters can be.