Hughes/MDD 600N or Explorer.
As another poster mentioned, uses the koanda effect of air blown through
a slot in the tail boom.
Because there is not tail rotor, you do not hear the beating effect of
the interaction of the main rotors and the tail rotors, only the turbine
engine(s) when it gets close.
I happened to be at the Mesa Airport when one of the first 600N's was
flitting about back in 1991.