Whistle Squeak sound braking – Ford Expedition V8 2008

Thanks in advance for any help.

I recently had my brake pads replaced thinking the whistle sound was coming from worn out pads. Not the case. It mostly happens when I apply pressure to the brakes in reverse gear.

I included a video of the sound. Brake fluid level is fine.

Could it be brake booster? Brakes? Some kind of leak?

Brand: Ford
Model: Expedition
Year: 2008
Engine: V8