If the brakes start to make a squealing noise, it could be caused by the brake pads. The brake pad has a steel backing that also has friction material attached to it. When you apply the brakes, it will produce hydraulic pressure that will cause the brake pads to clamp down on the rotors. This will then create the friction needed. When the friction of the pads works against the rotors, it will help to slow and stop the vehicle. When the material on the brake pads wears down, it is an indication that it is time to change the brake pads. When the brake pad is worn down, a small steel spring clip can make contact with the rotor. This will then make the high-pitched squeal that you might hear.
The brakes squealing can also occur when the brake calipers stick when the brakes are applied. This is because the brake pad is in constant contact with the brake rotor. This will produce an excessive amount of friction and heat. The brake pads will overheat and start to harden. This will result in the rotors becoming glazed. The squealing sound you hear could be a result of the harder surfaces having a connection with one another. This can then result in a reduction of braking power as the brakes squeal. The brake pads should be replaced, and the brake rotors should be inspected for any damage they might have.
No matter what sound you might hear when you apply the brakes, it is important to bring the vehicle in for an inspection. Since the brakes are what allow you to stop, it is important to keep them in the best condition possible.