06 Prius - Loud Electric Engine Noise

For about a week now, I have noticed that my '06 Prius is making a LOUD rumbly whine (at about 140k miles), almost like an exhaust leak. It's coming from the driver side of the engine compartment and only makes the noise when the electric engine is engaged (providing power to the wheels or charging the battery - this according to the in-car display). It's most noticeable when the car is in park. It's loudest when my car is cold, but it's still there after my 1.5 hour morning drive.

Thoughts/suggestions?