although i havent heard the noise in ur car it sounds exactly the same as what i have, only started a week ago after a long drive, i have and rb25de box on and rb20det, my mechanic seems to think it isnt a huge issue and is possible something to do with the gear selector. will be interesting to see how urs turns out.
by the way how do i check the gearbox oil level?? i have been looking for a dipstick or something and no luck, could even be low oil?