Fine, except the noise in my car is coming from the FRONT ring and pinion (or gearbox, hard to tell).
So, my NEW WRX (ahem) also has this noise, at 16 miles.
What does that tell you?
My RS did not have this noise.
One final thought: it might be the 1.1:1 reduction gear at the back of the box, overdrive gears like that sometimes make a noise.