metal grinding noise after changing back brakes

Did you change the pads and rotors? How about the parking brake shoe? Those have a bad habit of rusting and falling apart under the rotor.
Either way you will want to look them over and see if there is something assembled wrong or maybe the fronts are also worn.

Also if the caliper bracket bolts and slide pins aren't perfectly tight enough then the caliper and/or bracket will rattle around. It could sound like a grinding sound if it starts touching the rotor. Seen it happen.