they probably are going to make them buzz like a normal car.. a little like a stationary fuel powered car. The faster the car goes the louder the buzz.
I can imagine if they all buzz at the same frequency we'd all go crazy!
Current cars are good for blinds, because they are louder the faster they drive, and near to silent on standby.
To not go nuts they'd have to create a pitch variation in speeds, not to get the person in trance.
The car can not sound like a police or firetruck alarm, and the system needs to be cheap and efficient.
Most likely the best system is one that's mounted on the wheels,and captures some of the magnetic waves of the electromotor, and converts that to a sound (eg: 1 wheel rotation per second could translate in a 400Hz sound).