There are a variety of reasons why a refrigerator makes noises. One of the most common sounds is a knocking sound, which is caused by the circulation of refrigerant while the refrigerator is running.
Among the other common sounds is a hum in units with an icemaker. This sound comes from the icemaker filling with water. Clicking, another common sound, has a couple of possible explanations. The first possibility is the defrost timer turning off. A second possibility for a clicking sound is the temperature control turning off and on. In units that have a water dispenser, a buzzing sound can be heard when the dispenser is in use.