What causes the brakes to make a high pitch "squeek" noise?

I have a 2004 Chevy Malibu sedan.

This generally means that you need to have the brake pads changed. Would suggest you take your vehicle to have the brake pads looked at before they get too worn and cause serious money to repair the brake system. The noise is just a reminder. The louder it gets the more money it will cost. If you just had your brake pads redone - you may need some special fluid sprayed on the brake pads to quiet them as they are new and sometimes can make noise.

Are you hearing the noise from the brake pad sensors, indicating the pads need replacing?

If you need to replace the pads, which is most likely:

. Use ACDelco OEM pads.

. Apply Squeak Relief or suitable anti-squeal compound to shims.

. Lubricate caliper pins with silicone grease or Squeak Relief.

. Examine discs. Don't allow discs to be automatically turned on a lathe, which can result in lateral runout (LRO) problems. It's best to use a ProCut machine, which virtually eliminates LRO, if the discs truly need turning.

. Change brake fluid, with an ABS-approved product:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-46563-5.html#post619000

How many miles are on the car? Have you ever had the brakes replaced? The pads may just have some glazing, which would cause a squeaking sound. Or like Tx said, the pads could be worn down to the wear indicators, which also cause a squeaking sound.