Brake squeal often happens during the first few hundred miles of use as pads bed in to a new or used discs.  It’s best to wait until you have about 1,000 miles (1,609km) before determining if there is a noise problem with your pads.  Some pads have metal wear sensors that when the pad material gets low the wear sensor(s) make contact with the disc causing them to squeal.  This is to let the driver know that their pads are low and need to be replaced.  Loosely fitted pads in the caliper will make noise.  Debris caught between the brake pad and disc.

Loose or missing brake hardware such as anti-rattle clips and/or shims.  There are steel springs and pins which allow the pads mounted in the brake calipers to move freely without rattling and vibrating excessively. However, these pins and springs wear and lose their tension over time resulting in binding and squealing.