I have exactly the same problem as this. Do you only get this noise when you are in gear? Mine is doing this only when in gear, doesn't matter which. No noise in neutral or even when coasting. Seems like its only when pressure and weight is applied to through the gearbox. If I match the revs when in gear going down a hill so no weight or pressure is going through the gearbox the noise stops. It was a bit of a conundrum to everyone I took it to, no one could say exactly what it was. But the theory is that it could be the diff, for sure gearbox problems. I will be replacing but need to find a reasonable price for one. Does anyone know of a place that does good boxes for good prices? Will be fitting myself. Thanks