I have a 2006 S40 T5 with 4200 miles on it which has developed a squeal from the rear brakes when backing up. I’ve had the car in twice (with a 3rd looming); and each time the pads were replaced, along with the rotors this last time in. The dealer can’t seem to find the problem, and I've done some searching around online and have found that while not widespread, the problem is not unique to my car; however nobody seems to have been able to identify the cause. Theories range from brake dust to the chemical make up of the pads themselves.
Has anybody heard of this problem and figured out once and for all how to correct this?