When our devices apply braking, it is normal for the motor to create a buzzing sound around 120hz (click here for sound). Our devices inject DC current into the motor and what you're hearing is the residual frequency of the line voltage. The AC is 60hz and when we turn it to DC it leaves a voltage ripple of 120hz. That is what creates the buzzing sound. As the braking torque is increased, the volume of the buzzing will gradually increase. Normally the sound is only a gentle buzzing. In some cases the sound can get loud but it's usually because a piece of the machinery (like a loose piece of sheet metal) is vibrating during braking. Or if the motor is enclosed in a cabinet, it can create a resonance chamber (like the body of a guitar). This only makes it loud when a part of the machine just happens to have a natural resonant frequency of 120hz and the sound can normally be reduced easily. If in doubt - contact us. We'd be happy to help.