2014 Chevrolet Impala - Grinds

Car makes a grinding noise when backing up whether it is hot or cold doesn’t matter. sounds like you are driving over a bed of gravel. have taken it to dealer 6 times, been told that it is the ABS system, hub, censor, and that they were not able to hear any noise at all, the car on the highway gets great gas mileage but driving local it pulls like the brakes are locking. The scraping noise is loud enough for it to be heard down the road. am taking it to a independent garage to see if it can be fixed. does anyone have an answer for me from personal experience?