GEnx320

It’s a pedestrian warning sound required by NHTSA. Toyota’s hybrids have it too, but each company has chosen their own sounds. You only hear it in reverse because it’s directed backwards towards the driver, but it’s also present in drive. https://www.reuters.com/article/us-autos-regulations-sounds/u-s-finalizes-long-delayed-quiet-cars-rule-extending-deadline-idUSKCN1GA2GV