Why is there a grinding sound coming from the rear passenger side when I drive slow?

I drive a 96 Ford Mustang V6. For the past couple weeks when I'm driving really slow, like between 10-20mph I hear a grinding sound coming from the back passenger side. When I go over 20 the sound goes away. Brake response is normal. It doesn't make the sound when I apply the brakes. I initially thought it was a bad rotor but I've had that problem before and it only sounded like that when applying the brakes(like a squealing). Could it be a loose bearing or some debris stuck up underneath somewhere? Worst case scenario and it's a bad rotor, estimate of how much to repair at Firestone where I have a store credit card?

Some debris or a rock stuck in the shield. Or shield got bent, just use a flat screwdriver to bend it back.

just jack up the backside of the car and rotate the wheels to see what's doing it. Check condition of the tire too.

Stick the back end on a jack and axle stands. You can both spin the wheels and hubs like this feeling and listening. It is also possible to drive the car like this while you listen.
Identify the issue before you chuck parts at it. Remember brake rotors and pads need to be replaced in pairs.