89 Isuzu trooper. Brake pads make chunking/chirping noises when backing up.?

(More like the way a pencil erasure drags across paper when you push it instead of pull it)Difficult to describe. Pads are hard and dry, so they are not crumbling apart. This is a highway used vehicle(never off road). Is there any anti squeal clips that are suppose to be on those units(maybe like they fell off) or am I getting early warning that pads need replacing? Friends car, so I can't look at it right now. Just getting her prepared for either buying pads(4wheel discs) It is the back 2 that are the noisy ones and that is in slow back up like out of a driveway onto a road. Going forward, no noise.