Motors exhibit noise. The important thing to know is how you distinguish normal noise from abnormal noise, and how to troubleshoot. Similar to a baby, a motor doesn't tell you when it's having problems or what you need to do, but the sound that a motor makes can tell you a lot. If a motor produces abnormal noise, the possible causes can be backlash, gears, excessive vibration, motor damage, or gearhead damage.
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.