Electric motors

Nothing to do with cars realy but still kind of interesting, just got a brand new bus in our workshop it has no engine just electric motors n a shed load of wires and the gear box is the size of a large lunch box (normaly the size of a car engine + gearbox in a hgv) i started it up and thers a quiet hum from the rear as I was approaching 40km/h there was still no noise, weird, ill add pics of engine & gearbox later