A freezer compressor is what pumps chilled Freon -- or another cooling agent -- through coils that line the freezer compartment. Usually, the compressor also cools the refrigerator compartment, but some units have dual compressors. The cooling agent inside the compressor is sensitive to being moved around, which can cause knocking noises. Also, loose components can cause knocking sounds. You'll need to check a few areas of the freezer and fridge to properly troubleshoot a knocking compressor.
Listen carefully to determine whether the knocking noise is coming from the inside or outside of the freezer. You may need to open the freezer door to clearly hear if the noise is inside or outside. Skip to Step 3 if the sound is on the outside or coming from behind the freezer.
Remove the evaporator fan cover inside the freezer if the sound is inside the freezer. Inspect the fan and remove stuck ice or debris. If the fan appears damaged or stuck, you should replace it.
Have another person tilt the fridge backward slightly so the front bottom is off the ground. Use your hands to raise or lower the leveling legs until the fridge is level. Use a carpenter's level to ensure the fridge is level.
Slowly lower the fridge back into place. Wait until the compressor comes back on. If the knocking is gone, the leveling corrected the issue. If the knocking persists, proceed to Step 5.
Pull the fridge away from the wall. Locate the compressor, a football-sized black or silver canister. Some fridges have back covers or grates; remove this with a screwdriver to access the compressor.
Locate the cooling fan if applicable and ensure there is nothing stuck in the fan and that the fan is secure. Sometimes the fan can clang around causing a knocking noise. If the fan is damaged or loose, you may need to replace it. In some cases, you can tighten the fan case screws to secure a shaking fan.
Tighten the compressor holding bolts or screws if the compressor seems to be shaking causing a knocking sound.