Refrigerators are typically somewhat noisy even when they run well, but a malfunctioning part, such as a compressor or a defrost timer, can cause extra noise within the unit. Even if the refrigerator still works, repairing the part can sometimes lead to better efficiency.
Refrigerators contain a number of moving parts, including fans and pumps, so most make at least some noise at times. If the excessive noise comes from the back of the unit, the problem is likely with the compressor, a pump or a timing component. Self-defrosting units have fans that distribute air between the refrigerator and the freezer; this fan can become loud if it malfunctions and might lead to excessive sound when a door is open.