The primary cause of brake system noise is vibration and all brake systems vibrate to one degree or another creating an initial noise. For the most part this initial noise is beyond detection to the human ear. In part this is due to the dampening effect which the larger mass brake components generate in the system. The key factor in dampening initial noise is the stability or rigidity of the brake system components. In most O.E. disc brake designs, dampening is assisted by (1) insulators called shims or noise suppression gaskets which reduce vibration between the disc rotor plate and the caliper brake pad contact surfaces and/or (2) guide plates or abutment clips and the caliper mounting brackets assure rigidity of those components.

When the initial noise in the brake system is amplified (instead of dampened) to a level which becomes disagreeable to the vehicle operator, this amplification is called a secondary vibration. The secondary vibration occurs when the initial vibration frequency reaches an audible level with a high enough frequency for the driver to hear and be concerned.

Friction materials are not normally the cause of a secondary vibration. It generally results from rust, distorted or loose components and/or the wearing or weakening of an original part. As components weaken or become fatigued from the heat and stress generated in the brake system, they no longer fit as tightly as necessary to maintain proper rigidity. Any vibration resulting from the pad backing plate to caliper contact points will result in a very audible, irritating noise such as squeal.

Many technicians believe that changing to disc pads with a “softer” (more forgiving) friction material will cure noise problems. In reality, changing to the “softer” material changes the balance of the brake system, and this results in a change in noise frequency to an inaudible noise level. The important tradeoff to consider; wear rate of softer pads is much higher, life expectancy is greatly reduced. Thermodynamics may be altered affecting brake safety and efficiency.

Understand vehicle brake balance:

With 70%-85% or more of the braking performed by the front disc brakes while 15%-30% is done by the rear brakes. Older rear wheel drive vehicles performed 55%-60% of braking up front and 40%-45% in the rear brakes.
Front wheel drive disc operating temperatures are much higher, resulting in increased glazing and wear.
Any brake system is capable of generating noise. While braking under these extreme conditions heat build up causes glazing of the friction surfaces (a thin surface hardening or flaking is formed on the surface of pads/shoes) and all these conditions can produce noise.
Brake noise may be heard during light brake applications, and usually when the brakes are cold. Make sure caliper and pads move freely and are lubricated. Make sure the guides or slides are not making contact with the rotor. Remove and sand friction on a flat surface and reinstall.
Repeated braking in heavy traffic or hilly driving conditions. During these very demanding operating conditions temperatures can reach 1000 degrees or more. At these temperatures brake squeal is a common occurrence due to a metallurgical structure change that occurs in the rotor or drum surface.

A dynamometer ("dyno") allows testing of friction materials (brake) in an in-jouse environment as an alternative to conducting vehicle testing. Inertia dynamometer testing, approved by vehicle and axle manufacturers and government authorities, simulates vehicle-driving conditions, thus producing results more quickly than road testing.

In the United States, the Department of Transportation issues a Federal Motor Vehicle Safety Standard (FMVSS), which involves dyno testing that test few performance aspects. An effectiveness test measures the efficiency of the brake and different pressures, the fade test indicates brake performance (or failure) at a high deceleration (high temperature) rate, and a recovery test is performed at a lower deceleration rate.

Asbestos or chrysotile asbestos was used for decades in the manufacture of automotive brakes and brake linings. It provided excellent heat and friction resistance, durability, and was inexpensive.

Even though asbestos has been banned for many commercial uses since the late 1970s, the Environmental Protection Agency (EPA) allows its use in materials in which the asbestos fibers are tightly bound, including disc brake pads, drum brake linings, and brake blocks. Asbestos linings and brakes are widely available in the aftermarket and are still used in high-end luxury cars.

When the asbestos fibers remain firmly bound in material, there is no danger; however, if the material is damaged or worn in any way, microscopic asbestos fibers can be released into the air. When you breathe them in, the fibers can become lodged in the lungs and, after many years, can cause serous diseases such as asbestosis (scarring of the lungs) and several forms of lung cancer, including mesothelioma.