Squealing brakes are quite common and they are just annoying and irritating. Squealing also indicate poorly set-up brakes.
Reasons:
1- The rotor or pads are contaminated.
2- Grease or oil on the brake pad and rotor.
3- The contact between the braking surfaces is misaligned.
Solutions:
1- Clean your rotors with a specific (oil-free) disc brake degreaser. An alternative and common remedy is alcohol. Use it to clean the disc rotors with a small rag. Avoid using spray lubes anywhere near a bike with disc brakes.
2- Clean your pads with sandpaper. If the grease has soaked through the pad, you might need to replace them. Don't use a degreaser or chemicals on brake pads.
3- The calliper is not lined up with the disc rotor, or the rotor is slightly bend which can be easily straightened with careful use of an adjustable spanner.
