Is this the sound that is reminiscent of an old steamer trunk squeaking across a floor on those miserable little wheels (pre-casters)?
Mine just started that under moderate braking a few days ago.
I have a standing game of phone tag with a local service advisor to get other warranty work done. It started as a decal replacement (both sides' MCA rockers) and has grown to include a nasty rattle at headliner/windshield before this.
He was denying a loaner for the decal work, but it may have steamrolled on him beyond that point.
2013 Mustang 227 | MT82 | MCA | Performance Pack
First Car: 1979 Mustang 2.3L Turbo MT, Indianapolis Pace Car
First New Car: 1997 Mustang GT 4.6L MT