Inverter or MG whine?

have you ever heard power lines buzz?... it's the current being pulled through the inverter.

others might be more specific...

 

Prius makes many noises. Try making the car IG-ON, where the dashboard warning lights are on and the car is not READY. Then listen to the inverter coolant pump which is behind the driver's side headlight.

Then make the car READY and leave the gear selector in P. When the engine stops running, listen to the inverter whine. Nothing is moving, so the noise that you hear is not coming from the transaxle.

Once you are underway and the drivetrain is moving, then you'll hear additional sounds coming from the engine and transaxle.

 

Any time you have AC currents and magnetic fields together, the conductors are moving within the magnetic field, at least microscopically. When that movement is large enough, and at an audio frequency, you will hear sound. That is how speakers work, and how many kinds of electronic power supplies whine.

In the absence of deliberate magnetic fields, electric currents create it themselves.