You want to try something quick?
Use a long screwdriver to pry the disk pads away from the disk gently. It don't have to be that much movement, about a thou or two. Too much and the brake fluid will overflow, so be aware of this fact!
In a nutshell, in the standard rear caliper is a piston on one side only, that under pressure from the brake peddle activates the brake pad directly under the piston. The complete caliper slides over to one side so that the other pad helps to clamp the disk in between. Release the brake lever and the piston releases the pressure (about a thou). You with the aid of your screwdriver are now attempting to relieve any pressure on the disk. Listen for any sound similar to your video clip.
REMEMBER to activate the rear brake peddle BEFORE riding or you may be in for a nasty surprise. A delayed braking reaction / response.
You may ending up having to service / grease the caliper pins (very lightly) that allows the caliper to slide(too much attracts dirt).
Last edited by Gert; 06-22-2018 at 11:59 AM.