Then you'd hear it 5 times per rotation, instead there's one "whistle" per rotation, which matches up with the movement of the follower.
https://www.youtube.com/watch?v=EyzIKO6FmDc&t=377s
You can see it very well here, it's the same on the Mi-24 except it rotates faster.
https://youtu.be/n3sMAm6D4cE?t=179
An example on the Mi-24, you can hear the sound every one rotation of the follower, not 5 times as would be the case if it was the blades producing the sound.