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If you cannot locate the source of the problem, contact a certified technician to diagnose and repair your Frigidaire.
If you own a Frigidaire that is making a knocking sound, it could be the result of a couple of different causes. Some refrigerators make knocking noises when cycling on or off, as the fan begins to spin or stop. Unless the fan assembly breaks, this is simply a mechanical phenomenon that you can live with. If you suspect your Frigidaire's knocking sound might be something else, there are other considerations.
Check your Frigidaire's levelness. Approach the appliance and stand in front of it. Grab both sides and attempt to rock it without lifting it. If there is a sway from front to back or from side to side, the feet are not level. Consequently, when the fridge cycles, it will rock, causing a knocking noise. Look under the Frigidaire and even the feet with a pair of pliers by turning them up or down until it sits firmly on on four feet.
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Move the appliance away from the wall. If you level the feet or find the feet are not the problem, it could simply be your Frigidaire is sitting to close to the wall. Even though it runs quietly and smoothly on a normal basis, there is enough vibration during normal use to cause a knocking sound. Straddle the refrigerator and pull it away from the wall. If you see scuff marks on the wall, this is likely the problem.
Look at the fan and surrounding assembly. Pull your Frigidaire away from the wall and unplug it from the power outlet. Open the rear access panel with a screwdriver and locate the motor and fan assembly. Shine a flashlight into the unit and attempt to turn the fan manually. If it bumps against another component, this is the root of the problem. Straighten the fan blades, if bent, with pliers or replace the fan.