I am getting a metal on metal noise when braking in reverse after car sits for awhile, mostly when it is humid, damp out. The mechanic did not hear noise but front brakes were low so I had them replaced. No sound all day (dry out) then later it started to mist/snow and noise was back???
Metal Noise on 2008 Hyundai Elantra
Your mechanic will have to remove all wheels and re-inspect the brakes. Could just be the material the brake pads are made from but safer to get it checked. If it is making a noise that is due to metal to metal contact it will leave a visible mark on something.