When the brake pads are worn the rivets will actually touch the brake rotor when the brakes are applied. The brakes should be changed immediately.
check your wheel bearings
The squealing sound you hear when your car brakes are wearing out is referred to as a "squealer." This indicator is a strip of metal that when the brake pad wears down makes contact with the rotor making a metal on metal noise.
Some manufactures use a wear indicator that causes a squealing sound when the pads need to be replaced.
Dust, unevenness.
Usually either dirty braks or pads getting very low and needing replacement, the squealing is usually a warning, do your brakes or ruin your disks or drums.
Excessive velocity
Turning the steering wheel farthest left or right for a few seconds will cause a loud squealing.
The most common cause is slipping belt
The cause of the squealing sound is because you are turning the steering wheel completely to the right or left
the brakes or the emrgen see brakes
ABS brakes are not spongy. If you have spongy brakes the reason is that there is air in the system. You need to have your brakes bled.
If the brakes are too loose that could be a result.