Ford Expedition Whistle Squeak Sound when braking in reverse ?

What is your question?

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?