What I need you to do is check is this. There are sensors on the brake pads that are designed to notify you when the brake pads are low and in need of replacement. THe steel sensor touches the rotor and makes an annoying squealing sound (metal on metal). Check yours to see if by accident it has been bent and making contact with the rotor. Also, check the backing plate behind the rotor to see if it is bent and rubbing the rotor. Finally, if everything is good, they make a product that is a brake silencer. It is a liquid that you put on the back of the brake pads. It dries quickly and absorbs the vibration from braking stopping the noise. I'm not permitted to provide brands. We are not here to permote products. But, if you contact a parts store, they will know what you need.
Let me know what you find or if you have other questions.