Cause of Huey blade sound

Over on the rec.aviation.military forum a discussion got
around to the doppler effect and/or helo rotor blades, and
the following exchange took place:

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if the air reaches M 1.0 going over the
thickest part of the airfoil, bang!. Whop, whop, whop . .

Is that what gives the Huey its distinctive (not to say
spooky) sound?
I always assumed it had something to do with the fact that
it was
turbine powered, the blade downwash acting against the jet
blast. No?

No.

If it's just a matter of blade speed, why does no other
helo sound
like a Huey?

I've read an explanation somewhere; IIRR it's an artifact of
the two-blade teetering rotor design, but I'm going to ask
over on r.a.r. The four-blade Hueys (Model 412) now
available don't make that sound.
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Can anyone confirm (ordeny) my memory as to what causes the
Huey's sound? And if I'm correct, does this only happen on
Bell's teetering rotor design for the Huey, or is it also a
feature of the Model 47 as well as the Hiller UH-12,
Robinson etc.(or would be if they had large enough diameter
blades to get the tips up near supersonic)?

TIA,

Guy