This
problem has been covered quite a lot; the noise youre hearing should be happening in 1st-4th gears only, and will start momentarily after coming off the throttle and pretty much all the way down the rev band whilst on overrun.
The bearing on the layshaft is poorly constructed, and whats happening (to the best of my knowledge) is that the layshaft bearing is spinning in the gearbox casing.
Over time (mine has been doing it for at least 50,000 miles), the casing will wear out enough that the layshaft could have too much play in it - I would imagine the failure to be pretty bad if it did happen. The casing can be 'shimmed' in such a way that the layshaft bearing should remain fixed and not rotate in the casing.
I've heard quotes from £400 to £800 all in to have the box removed and refurbished - you could of course swap with a second hand box but that would be almost guaranteed to suffer the same issue. Your best bet, depending on mileage, would be to run the car until the clutch needs replacement (100-130k miles by my guess) and have it done while the box is out then.
Alex