Squeal

By far the most common suspect is a brake wear indicator. Most manufacturers have them, and GM has used them since the 1970's. The clue usually is there is something you can do to make the noise stop, and that will vary from car to car. Most commonly the noise will stop when you apply the brakes lightly. That is because the brake pads turn a tiny amount; just enough to let the "squeaker" touch the rotor, then they straighten out when the brake pedal is pressed, and that pulls the squeaker away from the rotor a very small amount. On other cars, the squeaker can be on the other end of the pad, and the noise will occur during brake pedal application, and stop when the brakes are released. This noise is easiest to hear when driving past a building and the noise echoes off of it.

There are a few other causes of high-pitched squeal, but they are all related to the brakes. This is a good time to have the brake system inspected. Your mechanic will measure the thickness of the rotors to see if they are worn past the legal minimum thickness. New replacement rotors today are so terribly inexpensive that most shops just replace them at every brake job. That is less expensive than buying the consumables for the brake lathe, and paying the mechanic's time to machine old rotors, and that translates into lower cost for you.