Nissan has said that the Leaf will actually have two different sounds - a "chirping" noise, heard here, that the vehicle will emit when traveling at low speeds in reverse and the bizarre, jet-like sound that will be made at high speeds, which you just heard in the video.
The automaker added the noise to the Leaf due to that fact that electric motors are extremely quiet. So quiet, in fact, that they are almost completely indiscernible on the road when mixed with the ambient noise of the vehicle's environment. This creates a major safety concern, especially for blind people who rely on being able to hear cars, or the lack thereof, in order to safely cross the road and walk along streets. Thus, the new "Leaf sound" is employed as a way to alert pedestrians of the vehicle's presence.
We're very happy to see that Nissan has taken this shortfall of EVs to heart and has put a lot of time and effort into the remedy. We're sure that this will benefit more than just the visually impaired, as well, given the amount of times we do not heed our mothers' advice by looking both ways before crossing.
The Leaf is actually not the first EV to be fitted with a sound for this purpose. Within this past year, GM worked with several members of the National Federation for the Blind in order to develop a sound for the Chevy Volt, the General's new extended-range electric vehicle.
Check out GM's version here and sound off on which you think is the better option (no pun intended).