I don't know if helicopter silencing is real or not either. I suppose it's possible to engage a quietER mode, maybe by adjusting the blades' angle of attack or some such, but I doubt it would make much difference at close range. A big portion of the noise of helicopters comes from the tail rotor and the intersection of sound waves between it and the main rotor. Some of the tailless designs can probably be made pretty stealthy, but AFAIK these were good old Blackhawk choppers.