No matter what type, brand or age of your refrigerator, it will make some noises. Refrigerators are simply noisy appliances because so much is going on inside and around them. The refrigerator's popping noises may be nothing unusual. A refrigerator makes sounds you could classify as popping throughout the day.
One of the most common reasons for popping noises in your refrigerator is thermal expansion. Refrigerators are generally cold appliances, but they change temperatures regularly as the cooling cycle turns on and off and as the defrost system does its job of melting frost off the evaporator coil. These slight changes of temperature will make the plastic and other materials in the refrigerator expand and contract as they get warmer or cooler. As the temperatures drop or rise, the expansion of these materials may pop and sound unusual. But if you spend a lot of time around the fridge, you'll realize the noises are simply part of the everyday workings of the appliance.
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Some popping noises could be caused by normal vibrations within the refrigerator. The powerful compressor motor can cause enough vibration in the unit that plates sitting on the shelves inside may tap together and cause what sounds like a continuous popping from outside the unit. If the floor is uneven under the refrigerator, the popping may be caused by the entire unit moving back and forth. Sometimes, loose objects like change or other random items may be on top of the refrigerator and vibrating, causing a popping noise.
When the water inlet valve on the back of the refrigerator opens to let water into the ice maker or water dispenser system, it makes a snapping or popping sound. This will only happen from time to time normally. If the popping is continuous, check to see if the water supply has been interrupted. If no water is coming through the valve, it could continue popping over and over and could be quite noisy.
If you notice popping specifically after the defrost cycle has run, it is usually because of the evaporator coil getting cold again. But if the defrost timer is malfunctioning and is stuck in the cooling cycle, it could continue to make the coil colder and colder, perhaps causing continuous popping until it is repaired.