In the movie, the special "Blue Thunder" helicopter has a "whisper mode" which renders it almost silent.

This is an impossibility, regardless of how many tales of "black helicopters" or the equivalent have been placed in movies, tv, etc. You cannot move the amount of air necessary to fly/hover a helicopter without making FAR more noise than a "whisper mode." True, you CAN REDUCE the amount of noise by adding more main rotor blades and slowing their rotation down, but you CANNOT choose between a "regular" and "whisper" mode.

Reason I know: I flew helicopters for almost 20 years.