A: There are two kind brushes we normally use: the metal brush and carbon brush. We choose based on the speed, current and lifetime requirements. For quite small motors, we only have metal brushes while for big ones we only have carbon brushes. Compared with metal brushes, the lifetime of carbon brushes are longer since it will reduce the wear on the commutator. But for low speed motor, the carbon dust might not be able to burn in time which will cause failure too.
Q: How about the noise level of your motor and do you have quiet motors?
A: Normally we define the noise level (dB) based on the back ground noise and measure distance. There are two kind noises: mechanical noise and electrical noise. For the former, it's related to the speed and motor parts. For the latter, it's mainly related to the sparks caused by the friction between the brushes and commutator. There is no quiet motor (without any noise) and only difference is the dB value.