Your brake pads have a metal scraper on them and when the brake pads get low they start scraping when brakes are applied to warn you your brake pads are worn out. There is a slight possibility that you have a ridge on your brake rotor and they need to be machined.
RepairPal Certified shops can diagnose and repair your 2006 Ford Five Hundred. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.