If you would like a thorough technical description of brake noise or to learn about the physics of brake noise, you might be interested in reading “Introduction to Brake Noise and Vibration” by Weiming Liu & Jerome L. Pfeifer at Honeywell Friction Materials.*

Here’s what Liu & Pfeifer have to say about Brake Squeal:

Brake Squeal

Brake squeal is a phenomenon of dynamic instability that occurs at one or
more of the natural frequencies of the brake system.

The excitation comes from the friction couple.

The disc rotor is acting like a “speaker” (sound wave radiated from the
rotor surfaces).

Pads and rotor coupling has major impact on mid to high frequency (4 ~
16 kHz) squeal.

Low frequency (1 ~ 3 kHz) squeal typically involves caliper, anchor
bracket, knuckle and suspension, in addition to pads and rotor.