EU legislation, introduced July 2019, requires all new four-wheeled electric and hybrid vehicles to have acoustic vehicle alert systems (AVAS) that make an audible warning sound when travelling at speeds of 12mph (20km/hr) or less. They cannot be switched off by the driver. The sounds are not standardised so each manufacturer is developing their own sounds.
Two example sounds from Nissan Electric:
This is the sound the electric Leaf makes when driving at low speeds.
This sound will play when the car moves backward at low speeds.
Image source: https://www.automotive-iq.com