Q: My car has brakes that often squeal. My repair shop says they can’t see any problems with the brakes and they can’t duplicate the noise I’m hearing. Are the brakes sure to stop when needed? The noise is embarrassing and shouldn’t be there.

A: The cause of this generally harmless but annoying sound is likely vibrations of a disc pad or possibly the caliper within its mounts. It’s important to mention that worn-out disc brake pads can also be noisy, due to metal-to-metal contact between the disc pad backing and rotor (the friction material is worn away). Or, it may be that a metal wear sensing tab is rubbing against the rotor (typically a constant or pulsating squeal that occurs some or at all times and changes with brake application). Brake noise can occur due to brake pad composition or condition, the pad vibrating in its nest or dry/unlubricated brake caliper slide pins. It’s also possible the pads have been overheated at some time and the friction material has become glazed. The shiny and hardened friction surface skips and bounces across the brake rotor surface.

Brake rotors can also develop hard spots and surface irregularities that cause noise. Also, during brake service, improper machining methods leaving a rough surface can make matters worse. A dull tool bit or impatient/fast cutting speed can cause a rough surface that promotes pad vibration. The surface finish should be less than 60 microinches for best results. Also, cheap replacement rotors may not have the best metallurgy or harmonic suppression features.

Disc brake pads come in many formulations. When choosing brakes, ask for a pad composition that prioritizes quietness over long life and buy the top-of-the-line grade. If the disc pad employs springs, clips or shims to dampen movement, it’s important these parts are present and in proper condition. Replace them if questionable.

There are a few noise reduction techniques that can be employed when installing the pads, such as adding a layer of special RTV adhesive/cushion to the pad back. A very tiny application of high temperature grease at the edges of the pad’s metal backing, where they contact the brake caliper, and the caliper sliding pins should also be performed. It’s really important grease does not get onto the disc pad’s friction surface. Chamfering (tapering) the leading and trailing edges of the pad’s friction material may also help keep things quiet. Premium quality pads typically come this way.

Your best chance of quieting the squeal would be to renew the pads with a premium grade product (generally includes new shims, clips, et cetera) and skillful machining of the rotor surfaces, along with applications of pad backing adhesive and grease mentioned above.