How to Fix a Squealing or Grinding Brake Noise

A noise from the brakes on a car is one of the few mechanical complaints where stopping is the correct immediate response. Brakes that make a noise are telling you something, and the noise itself usually identifies what is happening. The distinction between a squeal and a grind matters, because one is frequently a warning about a component that is about to fail and the other is the component failing.

This guide covers the sounds and what they mean.

The Two Sounds and Why They Are Different

A squeal is a high-pitched sound produced by vibration, and it usually means a component is moving against something it should not be touching. A grind is a harsh metallic sound produced by two surfaces making metal-to-metal contact, and it means that contact is already happening.

That is the whole distinction. A squeal is early. A grind is the same problem a little further on. Because the transition happens somewhere between them, a brake noise that has been present for a long time and has become a grind means the pads reached the limit of what noise could tell you, and the repair has grown accordingly.

Squealing Causes

Worn or Glazed Pads

The most common cause, and the least serious. A pad that has worn thin develops a vibration between the pad and the caliper, which is the squeal. A pad that has been overheated and glazed develops the same sound, and glazing also reduces the friction so the vehicle needs more pedal travel to stop. Both are a normal consequence of use rather than a defect, and the noise is a prompt to check the depth rather than a fault in itself.

A Dirty or Unevenly Worn Disc

A disc that has surface rust, or one with a groove, or one that has been machined unevenly, will produce a vibration that the pads amplify into a squeal. A disc that has been part of the friction surface for a long time without ever being machined develops a shallow groove that the pad sits in, and that groove creates a resonance. Machining the disc in most cases resolves it, and machining is substantially cheaper than replacement.

Failure to Grease the Pad Backing Plate

The most common mistake during a pad replacement, and the one that most often causes an immediate squeal. When the backing plate does not have a film of assembly lubricant or brake grease on the contact points, it rubs against the caliper piston and the abutments instead of sliding, and that produces a very loud squeal. The fix is to remove the pads, clean the contact surfaces, apply a thin film of the correct lubricant, and refit. It is a five-minute fix that people frequently pay a workshop to do, and it is covered in our guide to replacing brake pads.

A Bent Caliper Bracket or Abutment

A bracket that has been bent in an impact, or an abutment that has corroded, changes the alignment of the pad and causes uneven pressure. This produces a squeal that persists regardless of lubrication, and it requires correction rather than lubricant. Where the bracket has been bent, machining the disc to match may work as a temporary measure, and the correct fix is the bracket.

Dry or Rusty Shims

A pad fitted without its shim, or with a shim that has corroded, changes the pad's behaviour and produces noise. Some anti-squeal shims are designed to reduce noise by decoupling the pad from the bracket, and fitting one that has rusted produces exactly the opposite effect. Replacement shims are inexpensive.

A New Pad on an Old Disc

A fresh pad on a heavily worn disc frequently squeals, because the pad conforms to the disc's shape in a way that creates a resonance. A run of moderate braking at moderate speed will usually bed it in, and if it does not resolve after bedding, the disc needs machining or replacing.

Grinding Causes

The Friction Material Is Fully Worn

The overwhelming majority of grinds. The pad has worn to the point where the steel backing plate is contacting the disc, and the disc is being cut by metal. This is the point at which the repair has escalated, because the disc has been damaged and may need machining or replacing as well as the pads.

The Wear Sensor Has Activated

A thin wire embedded in the pad squeals when the material has worn down to it, which is a designed warning rather than a fault. A wear sensor that has broken off and fallen onto the disc produces a different noise, a rhythmic ticking that varies with speed, which is regularly mistaken for something in the wheel bearing.

A Brake Pad Backing Plate Loosened

A pad that has separated from its backing plate, or a plate that has started to move, produces a heavy grinding and a rattle. Where the pad is loose in the caliper, the damage can happen very quickly because the loose piece acts like a hard block between the pad and the disc. Any noise that comes and goes with the brake pedal in a heavy way warrants stopping immediately.

A Faulty Caliper or a Stuck Piston

A caliper piston that is stuck produces a grind or a hum under braking, because one side is dragging. A seized caliper also produces a pull to one side and a hot disc after a normal drive, both of which are additional clues. A drag that is not addressed destroys pads and discs very quickly, which is why the other causes of a pulling vehicle matter. The related pull symptom is in our guide to why a car feels unstable at speed.

An Absent or Incorrectly Fitted Pad

A pad that has been left out of a caliper on one side will produce a heavy grind or a scrape, because the piston is contacting the disc directly. It is not common but it happens during work where a brake caliper has been removed and not properly refitted, and it should be caught before the vehicle is driven. Any noise that appears immediately after brake work is worth taking seriously.

Distinguishing the Source

Three observations locate it. When does the noise occur: on light braking, on firm braking, at low speed, at motorway speed, or only on the first application after stopping. Is it worse on one side, which can be checked at speed with the windows down or in a quiet car park with the wheels turned. And is it related to the pedal being pressed or released, because a noise that occurs on release points to a different component from one that occurs on application.

A noise that occurs on light braking and disappears under firm braking is almost always the pads and the disc, because light pressure lets the pad move. A noise that is worse the harder you brake points to the disc or a stuck caliper. A rhythmic ticking that varies with speed and is unaffected by the brake pedal is not the brakes at all, and is more likely a loose shield, a wheel nut, or a heat-exchanger mounting.

What To Do

If the noise is a squeal, book the brake pads checked rather than continuing indefinitely. If the noise is a grind, stop using the vehicle normally and have the brakes inspected. If the noise is a heavy metallic impact from within the brake, stop driving and recover the vehicle.

Never drive a car that is grinding, and never continue to use a vehicle where the brake pedal feels different from normal, however normal the rest of the car sounds. The pedal is a more reliable warning than the noise, and a change in pedal feel should be treated more seriously than any sound.

On the maintenance side, the habits that produce brake noise rather than preventing it are worth avoiding. Bedding the pads after fitting is in our pad replacement guide, and the driving habits that reduce wear are covered in how to extend the life of your brake pads. If this guide was helpful, you may also want to read why a car makes a rattling noise to check whether what you are hearing is actually elsewhere, and how to choose a workshop you can trust before booking brake work.