That high-pitched squeal some cars make when the brake pads get low is not a malfunction - it is a feature. A small metal tab on the pad, called a wear indicator or squealer, is designed to make that noise on purpose. Here is how this clever, simple warning works.
How the squealer tab works
The wear indicator is a small spring-steel tab riveted or clipped to the brake pad, set at a specific depth. While the pad has plenty of friction material, the tab sits clear of the rotor. As the pad wears down to a set thickness, the tab reaches the rotor and starts to lightly drag against it, producing a high-pitched squeal. It is a purely mechanical, no-electronics warning - cheap and effective.
Why it squeals when you are NOT braking
Interestingly, the wear-indicator squeal is often loudest when you are NOT pressing the brake pedal, and may quiet down when you do. That is because when you brake, the pad clamps firmly and the tab is held tight against the rotor; off the brake, the tab lightly skims and rings. So a steady squeal at speed that stops when you brake is a classic worn-pad signal. See why brakes squeak.
What to do when you hear it
The squealer is telling you the pads are near the end of their usable life - not gone yet, but time to plan replacement soon. Do not wait until it turns into grinding, which means the material is gone and you are damaging the rotor - see why brakes grind. Replace the pads as a full axle set with fresh hardware - the new pads come with new tabs.
Mechanical vs electronic sensors
The squealer tab is the mechanical type of wear indicator. Some vehicles instead use an electronic sensor that lights a dashboard warning - see brake pad wear sensors. Either way, the message is the same: time for new pads for your vehicle.