TAL, Lift the rear wheel off the ground....( know what I mean without explaining?). Place one hand at the top of the tire (12 O'clock) and the other hand at the bottom off the tire... (6 O'clock). Now push the top, pull the bottom. Then pull the bottom and push the top. There should be no movement. If there is, kiss those "center hubs" goodbye. Replace them. Now go check the front for the same movement.
OK, you have the rear wheel off the ground. Lower it, break the lugs lose, lift it up again and remove the wheel. Check the pad wear? See if the steel backing (where the pads are bonded to) has too much float between the caliper? This clicking when starting and stopping my be a high spot on the disk, that causes the pad to dance against the caliper every time the wheel rotates a 360 degree turn? Once you start moving, the high spot moves the pad back in the caliper and the noise goes away. It comes back again when you step on the brakes and then leave from a stop?
Those are two variables to think about.