Whistling brakes from interior only

[Guyguy]

Hi Guys I shave 2011 Expedition and I have Whistling noise and the brake pedal it kind of sounds like air Wolf if I press harder it goes away or softer however the vehicle stops fine haven’t brought it anywhere but those three years ago used it’s been great any ideas thanks

 

[99WhiteC5Coupe]

Hi Guys I shave 2011 Expedition and I have Whistling noise and the brake pedal it kind of sounds like air Wolf if I press harder it goes away or softer however the vehicle stops fine haven’t brought it anywhere but those three years ago used it’s been great any ideas thanks

Perhaps an issue with the brake vacuum booster?

What is an air Wolf?