This goes for drum brakes as well as disc brakes. The #1 cause of brake noise is the friction material itself. I’ve also come to learn that the backing has a lot to do with brake noise as well. If you purchase bargain brake pads or shoes, expect them to be noisy. These brake pads will stop your vehicle, but they won't stay quiet. It doesn't matter what kind of tricks you throw at them, or how much goop you put on them, they're still going to make noise. Cheap friction material is just that — cheap.
If you have to put anything on the brake pad to keep it quiet, it's not a good brake pad. Yes, some pads require shims and moly grease installed out of the box, but that's different than the myriad products out there designed to keep brakes quiet. If you've installed pads that aren't keeping quiet, getting rid of the noise might not be as simple as just replacing the brake pads, especially if they've been on there for a while. If that's the case, you might also have to machine the rotors before installing better brake pads. We'll talk more about the machining process later in the article.
Bottom line, if you're going to put brake pads on your vehicle, get quality, name-brand pads. You'll thank me later when you don't hear anything and your vehicle stops on a dime. You're already saving money by doing the work yourself, so why not invest in a good set of pads for the best possible result?
Noise can also be caused by more than just the pads themselves. Many brake pads come with a wear indicator. This indicator is designed to make contact with the brake rotor when the brake pads have reached their service limit. This is often a small metal tab that sticks out from the brake pad. When the pad gets worn, this metal tab comes into contact with the rotor and makes a very loud noise.
This is designed to make you notice that you need to service your brakes. You often hear this noise when backing up and the brakes are not applied. If you hear this noise, inspect the brakes; you might find that they're worn and in need of replacement.
Some high-end vehicles use an electronic indicator to warn the driver when their brake pads are at their service limit.
On these systems, you might not hear any noise, but you'll see an indicator on the dash that advises you to inspect your brake pad wear.
Another cause of brake noise is debris getting caught between the brake rotor and the brake pads. This often causes a scraping or sometimes even a grinding noise. The debris can be small rocks or small pieces of rust. This often happens after a wheel has been removed and something in the suspension or brakes has been serviced. Small bits of rust get knocked loose and end up stuck between the brake caliper and rotor. So if you or someone else has done work on your vehicle lately that required removing the wheels and you now have a scraping noise as you drive, remove the wheel(s), clean and inspect the brakes, then recheck for the noise.
A similar issue to debris is the splash shield coming into contact with the brake rotor. The cause of this is similar to what was listed above. Usually someone has gone in there and done some type of service and slightly bent the splash shield. This problem is easy to fix, and often, you don't even need to remove the wheel. Just grab the splash shield and bend it away from the rotor slightly. Recheck for the noise. If it's gone, move on with your life as a noise-eliminating hero.
Lastly, let’s talk about flash rust. This is something that often happens in humid climates. If you live in a dry climate, you might never encounter this issue. Flash rust occurs on the surface of the brake rotor (or brake drum) after the vehicle sits for a period of time. How much rust depends on how long it sits and the level of humidity in the air. After this rust forms, it can cause brake noise. It's usually a scraping or grinding noise when you're just coasting. Often, after a few brake applications, the noise goes away.
Sometimes flash rust can lead to a brake pulsation. This can happen after your vehicle has sat for an extend period of time in humid conditions. The pulsation comes about because under the brake pads the rust does not occur. This leaves a clean spot on the rotor.
As you apply the brakes, every time this clean spot comes around, it causes the rotor to shake or vibrate. If the rust is to this point, you might need to replace or machine the rotors to correct the problem. An easy way to spot if you have flash rust on your rotors is to simply look through the wheels. Oftentimes, you can see the rust forming right on the rotors. I will say that some rotors are worse than others when it comes to flash rust. Cheap parts yield cheap results. This is as true for brake rotors and drums as it is for anything else.
Here's a video about finding noises in the suspension. It also covers some tips on how to track down noise due to brake-related issues.