'Whisper Mode'

It's not possible to do what the film did due to the physics of a moving object like a rotor blade moving through the air at high speed, causing a pressure differential and producing lift / thrust like an airplane wing. Each blade also slaps into the disturbed air of the blade moving in front of it, causing more sound, and the cycle repeats hundreds of times a minute.

It can be modified somewhat, but cannot be made to sound like Blue Thunder's whisper mode. Note also the "slower" speed of the whisper mode sound in the film, which doesn't make sense, it's pure fictional. The sound of the gas turbine's air intakes sucking in, compressing and combusting air, combined with the rotor system transmission gearing whine, would also always be present.

Eurocopter (which by coincidence incorporated the company Aerospatiale / Sud Aviation that made the Gazelle model that Blue Thunder was made from) has developed a technology called Blue Edge, which reduces the sound footprint of a helicopter by putting specially-shaped tips on the blades.

You can see a demo video of it here:
http://www.youtube.com/watch?v=dBS1NRsYuF8

I wonder if they had Blue Thunder's whisper mode in mind when they named it Blue Edge.

As you read in the previous answer to your post, it isn't possible to do with helicopters (yet, anyways) BUT during the Vietnam War Lockheed did build a small aircraft for covert missions which according to tests "sounded like rustling leaves" when flying overhead. I'm sure you can easily find it on the web; as far as I know, it wasn't put into production......