Help need opinion on rear brakes!!!

For some odd reason ONLY when I'm going into reverse and slowly applying the brakes it makes a really deep hollow sound that vibrates through the car from the rear brakes. Now my brakes are fine what else could be doing this?

check the adjuster on the rear shoes. it may be hanging or if you recently had work done put together backwards...it happens

I checked the shoes, they're on the right way, I know the balance is off, but it makes quite the sound occasionally when I'm backing up. It vibrates thru the whole car.