General Maintenance & Repairs - Rear brake sound like it not grab the rotor ?

I dont know if this normal or not, but when ever it rains, where my car is parking up hill, I heard that speak noise look like the brake is not grab the rotor, look like it travel back a little bit, is this worm out rotor or brake ? thanks

If the sound goes away, (As the rotor dries out and rubs off any rust.) then this is a normal thing for a heavy car with wet brakes.