Most likely your brake pads have worn down to the metal indicator tab and it's scraping against the rotor. This makes the per wheel revolution screeching sound. To isolate it without taking the wheels off, drive at low speed and pull the e-brake, if it screeches, then it's the rear left. If not, then it's the rear front. Easiest way to identify is just look through the inspection hole on the caliper with the wheels off. It'll be pretty clear which corner's pads have worn down.