This annoying noise disappears after the car has been driven for 15 - 20 minutes, but I don't believe it should be happening at all! Dealership has changed brake pads, re-checked new pads, etc. Their "research" from GM says noise is "normal" -- I say NO. Any help or advice before contacting GM? Thanks!
Noise is generally caused by vibrations within the braking system. Check the mounting bracket on the vehicle is well secured, then the guide pins. Also, check the guide pins are lubricated: if they're stuck, they may cause toe-in or -out, or hit front- or rear-side first on the braking material.
Also check for grit on the pads and rotors, as these could make horrendous sounds. Also check for warped rotors. Usually happens after lots of emergency stops or if you're tailgating someone and apply the brakes very, very frequently.
Another thing to do is check if there's something caught between the rotors and the dust shield, as this can cause some ungodly sounds that will both scare children and make dogs chase you. Another thing to check, whilst on the subject of the dust shield, check the shield itself isn't rubbing on the rotor. This could cause the same results.
Also find out if it really IS your brakes creating the sound; Is it happening whilst braking at speed? Is it happening when you apply the brakes whilst cornering? And, as daft as it may sound, apply your brakes whilst you're stopped. If you have an ABS pump, the pump might be screaming at you for one reason or another.
Another thing to check is metal-on-metal contact. If your brakes have wear indicators, check them.
The two pieces marked as "Pad Clip" and "Spreader Spring" are meant to be there. If they aren't, get some new ones. If they are, check they're not rubbing on the brake rotor. Also check there's some anti-seize on where the METAL parts of the pads touch any other metal parts of the caliper assembly, but make sure it's a fine amount; Too much may get pushed onto the braking material on the pads or on the rotors themselves. It doesn't take a rocket scientist to figure out why that'd be a bad thing on something that works with friction.
However, if the pads are brand-new, they may need a while to bed in. If you look at the pads and they have a stripe down them, squeals are normal; These are to wear the top of the pad down a bit to allow them to 'mesh' with the rotors. If, for other people, you have older pads, they might have 'glazed,' to the point it looks like plastic. This means they're too old, or have overheated, and need replacing.
Good luck fixing the demonic braking noises you're experiencing!