This Oriental Motor article provides more information on these types of noises and what can be done to minimize the noise.  It's important to remember to isolate the motor from any other noise-making source, such as metal plates or loose bolts.  It's also very useful to have an extra motor in good working condition for noise comparison, so you have a reference point.

The following article looks at the latest news on servo motors, drives and motors. How do we differentiate between the definitions? What can design engineers find between a stepper and a servo motor? Also, a quick update this summer on the benefits of closed-loop motor control.