Metal on metal. The pad is gone and the rotor is paying for it. Stop soon.
Pads worn to the backing plate grinding into the rotor.
Stop driving soon. Every stop machines the rotor. Pads now cost less than pads plus rotors next week.
Reading about sounds only goes so far. The tuner on our home page listens in your browser and tells you what it hears. The app goes further: it names the exact fault, checks it against your vehicle, and tells you how urgent it is.