Brakes make many noises, and the pitch is a genuine diagnostic clue. A high squeal points to very different causes than a low grind or groan. Learning to read the pitch helps you know whether it is harmless or urgent. Here is the breakdown.
High-pitch squeal
A high-pitched squeal or squeak is usually vibration-related, not damage. Common causes: the wear-indicator tab telling you pads are low - see the squealer tab - glazed or cheap pads, missing shims/hardware, or surface rust on the first morning stops; see why brakes squeak. High and intermittent usually means pad or hardware, often fixable and rarely an emergency.
Low-pitch grinding
A low, harsh grinding or growling is the serious one. It typically means metal-on-metal contact from pads worn completely away, with the backing plate cutting the rotor - see why brakes grind. Low-pitch grinding that you also feel through the pedal means stop and inspect now; every mile damages the rotor and reduces braking.
Low groan or moan
A low groan mainly at low speed or when nearly stopped can be normal resonance from certain pad compounds, or surface rust and moisture - see brakes squeak in the rain. If it is occasional and the brakes work fine, it is usually cosmetic. Persistent or worsening groan deserves a look.
Match pitch to urgency
Rough rule: high-pitch squeal = usually pads/hardware, plan a service; low-pitch grind = metal-on-metal, urgent. For a full sound-by-sound guide see brake noise diagnosis. Whatever the noise, fresh pads and hardware cure most of them.