Diagnosing Brake Noise

Brake noise diagnosis starts with accurately describing the noise. Squealing — a high-pitched sustained sound during braking — is usually vibration between the pad and caliper or pad and rotor. Grinding — a harsh metallic scraping — is metal-to-metal contact from worn-out pads. Clunking — a single knock when applying or releasing brakes — is a mechanical looseness in the caliper mount, bracket, or suspension. Scraping — a constant sound that may or may not change with braking — is often a backing plate contacting the rotor or a stone caught between the backing plate and rotor.

A brake squeal is a vibration at an audible frequency. The pad acts like a tuning fork between the caliper piston and the rotor. Several things reduce or eliminate squeal. Brake pad shims — thin dampening layers on the back of the pad — absorb the vibration. Brake pad grease or anti-squeal compound on the back of the pad and on the caliper slides dampens the vibration. Chamfered pad edges reduce the leading edge contact that initiates the vibration. Rotor surface finish matters — a rotor with a rough finish from a dull or worn brake lathe can cause squeal. When a customer complains of squeal, verify the pads have proper shims, the slide pins are lubricated and moving freely, the pad abutment clips are in place, and the rotor surface is properly finished.

Grinding during braking means the pad friction material is gone. The steel backing plate of the pad is grinding directly into the rotor. Every second the vehicle drives in this condition, the rotor is being destroyed. Deep grooves are cut into the rotor surface. The rotor may be thinned below minimum specification. In extreme cases, the pad backing plate wears through and the caliper piston contacts the rotor — destroying the caliper as well. Grinding brakes are a safety emergency and a liability issue. Inspect immediately. Do not schedule it for later.

Drive at 30 mph on a quiet street. Apply light braking — listen for noise. Apply moderate braking — listen again. Does the noise occur only during braking or is it constant? Apply brakes while turning left, then while turning right — does the noise change? This can help identify which side the noise comes from. A noise that disappears with light brake application is often a backing plate scraping the rotor — the light brake application pushes the rotor slightly away from the backing plate. A noise that only occurs during the first few stops in the morning but disappears after — surface rust on the rotors from overnight moisture. This is normal and not a defect.