I had the same problem, only it was both of my front brakes. It was the most ghetto sound. I knew that my pads were brand new because the car dealership that I had bought the car used from had tried 3 different pads to fix the "problem". After living with the loud horrendous sound for about 2.5 months I finally took my car to a Toyota specialist that came highly recommended (NOT the dealership). They told me that I had no anti-squeal greese on my pad and I just had the wrong pads to begin with (they were too small for the caliper thus moving around and making the scraping/rap-rap sound?)
. Anyways, long story short... they hooked me up with TRD
pads and gunked this blue stuff all over the back of the pads and now they are just great and have been for over a month. All for around $90, screw the dealership!