"Since the ER3 is nearly silent, the crew put a sound system on the car to
produce an engine-like noise to make sure that spectators lining the course would be aware that the ER3 was coming down the track. This sound system was programmed to make a buzz or whine that was proportional to the speed of the car. It seemed to fool everyone as most people at the track were very surprised when I revealed that the sound was artificial. This was a big improvement over the annoying siren used on the ER2."
That's the funniest thing, speakers to fake the engine noise. I've had a similar thought about EVs, since they are silent, people can't hear them coming. I once saw a Prius going slowly by, and I couldn't hear it at all. Could potentially be dangerous since you can't hear it coming like a normal ICE car.