New brake pads on an automobile commonly cause brake noise. The new pads will take some time to wear in, or "bed", and a tolerable amount of noise during this time is normal. Other noises could spell trouble; investigate them further.
New brake pads normally require about 300 miles of city driving before they become fully bedded, or worn in with the rotors. A tolerable amount of noise---usually a light squeal---is normal during this period. You can accelerate the bedding process by performing a half dozen or so rapid slowdowns from highway speed to about 20 miles per hour and then driving the car without braking for 10 minutes or so to let the brakes cool. Perform the decelerations in rapid succession so the brakes do not cool between them.
Soft, noise-reducing shims placed on the backs of the brake pads are the most common, and the most effective, means of eliminating most brake noise. Pad installation jobs often omit the shims, or make use of low-quality shims, with the result that the new pads squeal. Installing new, high quality noise-reducing shims will often fix the problem.
Brake Pad Lining Material
Brake noise can result from the installation of new pads with linings made from a harder material than the old linings. A barely audible grinding sound during low-speed braking is normal for hard linings---especially when the brakes are cold. Hard linings will more likely produce brake squeal; use so noise-reducing shims.
Brake Pad Design
High-quality brake pads often have a chamfered, or angled, edge along the leading and trailing edges of the pad lining. On low-quality pads, the lining edges usually have sharp corners. Pads with sharp corners are more prone to producing brake noise---especially during the bedding period---and a change to pads with chamfered edges will often fix the problem.
Improper Pad Installation
Improperly reinstalled calipers, or pads not positioned firmly in the calipers, will often cause vibration and noise. If the brakes are noisy immediately after servicing, double-check the brakes to ensure everything has been properly assembled.