2006 Pontiac Vibe Noise upon start up

Based on the lack of additional symptoms, (and the fact you haven't crashed yet :)), I would tend to believe what you are hearing is rust. For many years, the higher quality brake linings have had metal particles embedded in them to help dissipate heat and reduce brake fade. The downside is the noise. On many cars, the noise is present for many miles until the linings warm up. When they warm up, the binders, (glue) that holds the material together softens and reduces the tendency for them to stick to the rotors. Think of dragging your fingernails down a blackboard vs. dragging your fingerprints. Nails screech; prints slide quietly.

When the linings heat up and soften, the embedded metallic particles aren't forced into the rotors so hard. Those particles can also develop surface rust from the humidity in the air. Rust causes squealing. If I'm right, the next time you think this is going to happen, hold medium pressure on the brake pedal, then back up from your parking stall. If there is no noise, you know the problem is nothing serious.

There are wear indicators that will make a similar noise, usually when you are NOT pressing the brake pedal, but the key word is "usually". Since you had the brakes inspected, you can forget about worn brakes as the cause, at least for now.

Also understand that brake linings have different compositions at different spots. It is real common for them to not have quite so much friction, (stopping ability), when they are new. After a short break-in period, they usually work better. As they near the end of their life, there is less mass to dissipate heat, so there could be a change in the way they respond compared to new linings.

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