Why do my brakes make a squealing noise sometimes when I apply them lightly at low speeds?
As described in some owner's manuals, the squealing noise is caused by high-frequency vibration of the brake pads against the rotating brake disc. Vibration is the unavoidable result of friction generated by the pads as the caliper clamps them against the rotating disc. Under average braking conditions, some brake noise is normal and cannot be eliminated.

Normally, the shims and the high-temperature grease between the pads and the brake caliper dampen and isolate most of the vibration. The level of vibration, however, is affected by outside temperature and humidity, by road conditions (mud, dust, and road salt), and by the condition or thickness of the brake pad material.

If the squealing noise is abnormally loud, have the brakes inspected and checked for pad wear.