Replacement gearbox

[Paulcos]

I rotated the wheels whilst the car was elevated off the ground and the dif is knocking quite noticeably in my gearbox and it will need replacing.
At least it seems to me to be the dif. Could it be anything else?? It knocks whilst steady cruising but is quiet under load and deceleration
The gearbox is an 02j exb on a 6n2 1.4tdi.
Thank you for your help