If you're driving around one day and you hear an awful screeching or grinding when you apply your brakes, don't ignore it. Among the possible causes are worn brake pads, which constitute both a major safety issue and a serious risk to your rotors. The following is a set of steps that you can follow in an attempt to self-diagnose the source of the problem. But unless you're comfortable working on your own car, a persistent screeching sound from your brakes almost always leads to a trip to the mechanic.
Recall the recent weather when you first hear the screeching noise. If the weather has been especially wet or rainy and you haven't driven the car in as much as 24 hours, the sound may just be the brake pads scraping some harmless surface rust off of the rotors. If this is the case, though, the noise should go away completely after about 10 complete stops.
Stop the car and get out to look at the wheels for a visual inspection of the brakes. If your brake pads are worn down to 1/16 of an inch or thinner, the sound could be the metal-on-metal scraping of your brakes wearing out completely. Some brake pads have small metal tabs built onto them, and these tabs scrape along the rotors when the pads become too worn, emitting a loud screeching noise. If a visual inspection suggests that either of these are the cause, replace the worn brake pads immediately.
Continue driving if you're still unsure of what the cause of the noise is, but drive with the windows rolled down. If you can, stick your head out the window a little bit as you make your stops. If the sound appears to be originating from the wheel well, there is likely a problem with the brake pads, calipers, rotors or a wheel bearing. If the noise sounds more like it's coming from underneath the car or in the very front, it may simply be a creaking suspension. Suspensions often begin to "settle" after years of driving, resulting in occasional noises which are usually not suggestive of major safety issues.
Take your car to a brake specialist unless you determine that the noise is related to a harmless problem like slightly rusty rotors or unless the problem is worn brake pads that you intend to replace yourself. Share your findings with the mechanic if you were able to pinpoint the source of the noise to a particular place.
