Porsche will have to add fake noises to hybrids like the 2013 Panamera. Porsche Starting in two months, hybrid and electric motor vehicles will be required to play recordings of conventional car-like sounds to alert pedestrians and cyclists to their approach.

The National Highway Traffic Safety Administration issued proposed rules today to govern the sounds, required under the Pedestrian Safety Enhancement Act of 2010.

The proposed rules specify the noise be recognizable as coming from a motor vehicle, and change based on the vehicle's speed and direction. The recordings would only be necessary while the vehicle is traveling at 18 mph or less.

The cost of implementing the new rules would be rather low: The NHTSA estimates it at $30 per vehicle, and that the extra weight of the speaker system would add about $5 in fuel costs over the lifetime of the vehicle. Both costs are insignificant.

The NHTSA estimates the rules could prevent 2,800 cyclist and pedestrian injuries over the life of each model year of the vehicles, and save 35 lives (a low number, as relatively few deaths are caused by low speed collisions).

While the extra noise may annoy owners who value the quiet nature of their fuel-efficient cars, the new sounds likely won't sound like internal combustion engines.

Per their request, automakers will have flexibility in deciding what sort of sound their car will make, providing them an interesting opportunity to create a sound that is unique to their vehicle. But it is likely most will go with a whirring noise that will fit within the NHTSA's rules.

The public has 60 days to comment on the rules before they are finalized; automakers will have about four years to equip all their hybrid and electric vehicles with the ability to play the sounds.