Skin caliper measurements are usually the least expensive method.
This sound indicates that you've worn down the brake pads to the point where they are gone, and you have metal on metal contact between the brake caliper and the rotors.
You look for a withered limb, a shorter leg, different size feet, a caliper.
brake caliper in position sheared, whilst taking Gwyn out for a spin.
New front brake hose routed directly to the front brake caliper.