This picture of  F82 M4 rear axle,,,the brake caliper and brake rotor are removed,,,,so all we see is the rear hub,parking brake shoes and backing plate,,,,the customer complaint about rear grind coming from rear wheels,,,,what happens is that little rocks get between the backing plate and brake rotor causing a grinding noise. So here is the old backing plate removed,,,,you see the vent holes on top,,,that’s where the little rock get in behind the brake rotor. Here is the new and updated rear backing plate,,,look at the vent on top they are not completely open. Ok old design. New design,,,,this repair is also on M3