Or mono trials brake squicks when you brake, which is normal, means tha brake is working fine, some times the rotor catches the corner of a brake pad and squicks when wheel spins slownly ( maybe it is not set up perfectly or rotor is a bit bent), had this on my brake - it is completely normal. Unless what forteh said it might be contaminated, but that is when you dont have enough braking power. Which is it? Also, you didn't oil your disc brake like silly people do?