Electric motorcycles have this problem, too. Last year a popular German motorcycle magazine ("Motorrad") tested electric motorcycles in their big mountain test where they bring almost all available motorcycle models from that year to the alps.
The loudest motorcycle driven at normal speeds was an electric one.
I really hope they can bring that noise down further.
It has to be either the planetary gear set or the chain - none of the gears are helical, so they're bound to be noisy. According to the specs the motor is a permanent magnet synchronous motor, which is normally mostly silent.
Electric cars are whisper quiet at low speeds; at higher speeds tire noise becomes prominent. I wouldn't be surprised if they intentionally add noise for safety reasons.
