Umm, there was an issue with the rear brake pads some time back. Found this in my files from an owner on one of the forums. If it's not brake-related then I don't recall any previous complaints of a similar noise.
I had a noise when the car was regenerating and brakes not applied. Seemed to be coning from the front not the rear. They replaced traction motor and another part. I can look up the exact parts if you think this may be your issue.