When your brakes make a high-pitched noise every time you touch the pedal, there are a number of possible causes, some of which may not even be problematic. In most cases, the issue will either resolve itself or can be fixed easily. First, make sure your brake pads are not worn down. Once they have only about one quarter to one eighth of an inch of material, a small metal rod will become exposed. It is intended to create a high-pitched noise to alert you that the pads need to be changed. Once you've ruled this out, you can start working to eliminate the squealing sound.
More often than not, squealing noises are the result of vibrations between different parts of the brake system - brake pads and rotors, calipers and mounts, or pads and calipers. Under certain operating conditions, the parts can begin to vibrate. These vibrations are similar to those created by plucking a guitar string. They can be dampened or adjusted to vibrate in a frequency inaudible to the human ear.
If you only notice the sounds when it's raining, the sound you're hearing could be associated with a thin layer of rust that may have formed on the brake discs. The sound comes from the brake pads rubbing against this rust and should cease after the car stops a few times. The same sounds can also occur if there is dust along the brake pads or discs.
The material of the brake pads may cause squealing, and you can try replacing the pads with a different type to see if that gets rid of the sound. A ceramic pad can be enough to alter the vibration frequency and give your ears a break. Ceramic pads are ideal, as they often include features designed to reduce squeaking such as chamfers. Chamfers are beveled edges on the pads that reduce the amount of initial squeaking when the brakes are first applied.
You can also consider placing shims between the caliper pistons and the brake pad. These parts can be affixed to the backs of brake pads and are designed to take up some space and alter the vibrations. They can be a good alternative to replacing the pads, especially when they are many miles away from needing to be changed. However, if the pads are too new, there may not be enough room for the shims.
If that does not take care of the issue, you should check the calipers. If the parts of the calipers are not properly lubricated, this can lead to squeaking sounds. You should also inspect the caliper mounts to ensure they are not loose or worn. If they are, this can allow the calipers to vibrate excessively while you're driving, which would result in squealing brakes.
While you're inspecting the brakes, be sure to take a look at the rotors as well. The surface should be smooth and in decent condition. If the rotors appear visibly damaged or worn, it is best to replace them. However, it is difficult to measure smoothness with the naked eye. You can try writing on the rotor with a ballpoint pen. A continuous line indicates the rotor is smooth. A rough rotor surface will cause the ink to appear broken up. If you have tried everything else but are still experiencing squeaking sounds, the rotor smoothness could be the culprit, and you can have them resurfaced.