Worst case scenario would be a bearing in the differential of the transaxle. Run it by a transmission shop and have them listen to it.
Do you plan on replacing the tranny?Update: Stopped by a couple of transmission shops, which said that they needed to have the car for half a day, scheduled beforehand, in order to diagnose. My personal mechanic had a look and used a stethescope to listen while the wheels were turning. He said it's coming from inside the transmission.
You stated that the car has 231K miles - do you know when the fluid was changed last? What is the condition of the fluid? How does it smell? Does it feel right? Check it against new fluid. Changing the fluid will not make the noise go away.Should I change the tranny fluid?