[Impala]

Never come across any posts relating to such a noise, apart from the grinding rear brake pads when reversing.

Is this with the car still stationary and the clutch fully depressed ?

Even if it was stationary and in neutral with the clutch pedal released, the input shaft to the gearbox would be turning at engine speed and may still turn somewhat as you depress the clutch. A grinding or grating noise when you then put it into gear would suggest a synchromesh problem with the reverse gear (if it has one for reverse), but replacing the entire gearbox would rule that out , so I wonder if anything in the gear lever linkage could be to blame.

 

[Impala]

This sounds like clutch drag (ie the clutch is not fully disengaging the drive) rather than a gearbox issue, and the syncromesh on the forward gears masks the problem. As there is no syncromesh for reverse, the gears grind unless you take other actions, as you do.

Have they tried replacing the clutch release bearing, actuator lever (perhaps it is flexing under load), checked clutch plate is free to slide along splines in drive shaft, checked/replaced clutch cable (if it has one, and checked outer cable is secured), master/slave cylinders, or even just adjusting the clutch ? I don't know if the T-Roc has a cable or hydraulic link between the pedal and the clutch actuator, so some of the things listed may not apply.

I would have thought this would be fairly easy for a good mechanic to diagnose and repair.