Many papers of brake noise research were presented before. Today it is required in brake design stage to predict the influences of countermeasure for every brake noise. For example, moan noise is sometimes caused by the countermeasure of low-frequency disc brake squeal. The countermeasure should be derived from the analysis for reducing both brake noises, but there is no paper of relationship research between each brake noise. This paper describes that moan noise is given by one of the dynamic instable solutions in low-frequency disc brake squeal, which is given by 4 degrees of freedom of caliper with suspension and brake disc. On the lower stiffness of caliper fixed on suspension, the vibration amplitude of disc is sufficiently small and the amplitude of disc is considered to be negligible. Then the equation of motion is reduced into 2 degrees of freedom, which gives the equation of motion in moan noise. This consideration enables us to reduce moan noise and low-frequency disc brake squeal at the same time.