After years of trying to make cars sound as if they were riding on air, engineers are considering how they might bring back some noise. They're trying to make some of them — those silent hybrids — more audible.

After years of trying to make cars sound as if they were riding on air, engineers are considering how they might bring back some noise. They're trying to make some of them — those silent hybrids — more audible.

But how? A team of engineers developing the Leaf, the forthcoming electric car from Nissan and a front-runner in the race for a mass-market electric car, have recently been presenting their ideas for artificial noises to government officials and focus groups.

Maybe CHIME NUMBER 22? MELODY NUMBER 39? Perhaps a futuristic whirring like the aircraft in "Blade Runner"?

As hybrids proliferate, some automakers are seeking to address concerns in the United States and Japan that the nearly noiseless vehicles may be so quiet that they pose a threat to pedestrians.