High frequency continuous tone when brakes are applied, this - single or multiple tones. No one enjoys squealing brakes!
Unevevnly worn brake pad surface condition caused by scored brake discs. Uneven wear between 'in-board' & 'out-board' pads. Disc 'run-out' cased by dirty /contaminated hub mounting. Inadequate lubrication of brake pa/brake caliper 'contact points'. Worn/missing shims. Worn or broken anti-rattle springs.
Fit new brake pads and ensure shims, pins and springs are fitted correctly. Check brake discs and replace if necessary. Replace and lubricate fitting kits with a product such as Mintex Cera TEC.