A small thing to check when your working at the rear calipers is the handbrake cables make sure the little arm can go all the away back to the stop by itself. The cables can loose the plastic coating over the wire and stick keeping them on if you move the arm forward you can unhook the end of the cable and push it in and back out with your hand to feel if they are free. The problem is that this fault is not obvious and may release slowly but can lead to premature pad/disk wear. Make sure the pads move freely on the guides even if you have to lightly file the metal edges. Winding the piston in make sure the rubber boot has not stuck onto the piston making it very hard to wind or even rip the boot. Your noise can be the outer and inner edge of the pad rubbing on the disk sometimes it's necessary to file the step created by the pad as it wears down the disk, failing that look for small stones wedged in or the floating slide pins that allow the caliper to move in and out they can seize and some places put the wrong size pins back in which can give it too much play. Use copper grease and good luck finding the problem
