From an NTSB
document:
Hybrid electric vehicles generate very little noise when operating under battery power, making them hard for pedestrians to detect. In December 2016, NHTSA issued a final rule establishing FMVSS 141, which sets minimum sound requirements for hybrid and electric vehicles. In February 2018, NHTSA published a revised FMVSS 141, based on petitions for reconsideration. Under the new safety regulations, hybrids and electric cars will be equipped with a device that emits a sound to alert passersby that the vehicle is running. Manufacturers have until September 1, 2020, to meet the requirement. This is a positive step by the regulator.