I had a Grand Am that did that and it was the brake hardware. The thin metal pieces between the caliper and pads. It made no difference on application. They lost their tension and let the pads vibrate. Cheap to fix.
/ HELP! - Squeaking noise coming from front wheel?
Another reason I do my own work is getting it done right. When you replace brake pads, you should use an adhesive like "disc brake quiet" to "glue" the pads to the bracket and piston so no squeaking when the pads slide around.
/ HELP! - Squeaking noise coming from front wheel?
I took it back to the shop and had them look at and take it for a drive. They could not hear anything, so the mechanic had me take him out for a ride. Once I pointed it out to him, he then could hear it. He took it back and looked at the wheel/brake area and did not see anything, so he greased it up and put it back together (all for free of course). The noise continued for a few minutes and then it stopped and has not been back since.