brake calipers: the assembly which has the brake pads and piston, mounted on the hub
it slides on metal bushes, greased and has rubber boots.
any extra movements or vibration in sliding can cause the bushes to wear out early and leaving the calipers loose for extra khar khar noise
the boots are usually torn out and very few parts shop has them, so if the grease is out, the caliper can stuck in its place and will push the brake disc from one side. leading to bad braking...