There are two types of squeal: one is a high-pitched shriek and the other is a low-pitched growl. A high-pitched shriek occurs during normal braking. It also occurs during light braking on vehicles with disc brakes. A low-pitched growl occurs just before the vehicle comes to a stop when the brake pedal is depressed hard.
Fluctuations in brake pad friction or shoe friction (due to hardening of the friction material).
Resonances in the brake rotors, drums, or backing plates due to lack of rigidity in those components.
When the brakes are applied, the friction generated by the disc pads and rotors cause the disc pads to vibrate.
The disc rotor resonates with this vibration, creating noise.
Some disc brakes use rubber-coated anti-squeal shims, and some use double-layered anti-squeal shims. Another method of reducing brake squeal is to apply disc brake grease to both sides of the inner anti-squeal shim(s).
When the brakes are applied, the friction generated by the brake shoes and brake drums cause the brake shoes to vibrate.
The brake drums and backing plates resonate with this vibration, creating noise.