A knocking sound coming from the refrigerator when you open the freezer compartment could be due to a few different reasons:
Expansion and contraction of materials: When the freezer door is opened, warm air from the surroundings enters the freezer, and the temperature inside the freezer increases slightly. This can cause the various components, such as the evaporator coils and other metal parts, to expand. When you close the door again, these components cool down and contract, which can create a knocking or popping sound.
Ice buildup: If there is ice buildup around the fan or other components inside the freezer, it can create resistance when the fan starts spinning after you open the door. The resistance can cause the fan to make a knocking sound as it pushes through the ice.
Loose components: Over time, some parts of the refrigerator, including the fan, fan motor, or other internal components, may become loose due to regular usage and vibrations. When the door is opened, the sudden movement might cause these loose parts to knock against each other, producing the sound.
Defrost cycle: Modern refrigerators often have an automatic defrost cycle to remove ice buildup from the evaporator coils. During this cycle, the refrigerator may make some unusual sounds, including knocking noises, as the defrost heater turns on and melts the ice.
If the knocking sound is infrequent and doesn't persist for an extended period, it's generally not a cause for concern. However, if the noise becomes constant or is accompanied by other issues like reduced cooling performance or unusual odors, it's best to have the refrigerator checked by a qualified technician. Regular maintenance can also help prevent excessive ice buildup and keep the components in good working condition, reducing the likelihood of unusual noises.