Why do brakes squeak, and does it mean they are bad?

Brakes make noise for several reasons and only some of them are problems. A light squeal on the first few stops of the morning is typically a thin film of surface rust that formed overnight on the rotors, and it disappears once the pads scrub it off. Certain pad compounds, particularly harder semi-metallic and ceramic formulations, squeal more than others as a function of vibration frequency rather than wear. Dust, road grit, or a small stone lodged against the rotor produces noise that comes and goes.

The sounds that demand attention are specific. A consistent high-pitched squeal that appears at most stops usually means the wear indicator, a small metal tab designed to contact the rotor when the pad gets thin, is doing its job, and that is a warning by design rather than a malfunction. A grinding or growling sound means the friction material is gone and metal is contacting metal, which destroys rotors quickly and turns a modest pad job into an expensive one, so that is a stop-driving-and-service-it signal. Also treat as urgent: a pulsing or vibrating pedal, which suggests warped rotors, a soft or sinking pedal, which can indicate a fluid leak or air in the lines, and any pull to one side during braking. Brakes are the one system where deferring maintenance has an immediate safety cost rather than a delayed financial one.