If the car is standing still the gearbox output shaft is also standing still. If the clutch is fully disengaged the input shaft should also be standing still and there should be no speed difference between the gears and the synchro rings, so it shouldn't be able to make a grinding noise.

If you press the clutch with the car in neutral, then start the car and put it in first gear, does it also make the grinding noise? It could also be that the pilot bearing is dragging the input shaft with it.