Hi Scotty, recently bought a 2010 scion XB auto with 144k miles. while the car is running on the road I hear a humming sound like these superchargers or electric motor, coming out from the engine bay. Check the play on all wheels it's fine so it's not the bearing going out. check the car voltage while running it was within specs so not the alternator neither, then I thought might be the transmission since the original owner never change the fluid since Toyota now advertised it as "lifetime fluid", however, the car shifts fine with no issues, furthermore, I have replaced the transmission filter, changed transmission fluid twice, still same sound while the car is running on the road (no humming sound when the car is parked and rev up).

what could it be? thanks so much in advance!