Excessive frost on the evaporator can interfere with the rotating evaporator fan blade, causing a loud buzzing noise inside the freezer compartment. If you hear buzzing and the noise gets louder when you open the freezer door (indicating the noise is coming from inside the cabinet), check for excessive frost inside the freezer. If you find excessive frost inside the freezer, find and repair the cause to eliminate the buzzing noise.
A leaky door or lid gasket can cause excessive frost to build up inside the cabinet and on the evaporator. Replace the door or lid gasket if damaged. Manually defrost the freezer by safely storing food and unplugging the refrigerator. Prop the freezer door open and allow room-temperature air to circulate through the freezer cabinet to defrost the evaporator. After 24 hours, close the freezer door and plug the freezer back in to restore cooling. If the noise resumes, then you may need to replace the evaporator fan because its blade is damaged.
If you find frost concentrated on the evaporator cover, the automatic defrost system likely isn't working. A broken defrost heater, failed defrost timer or bad defrost bi-metal thermostat could allow excessive frost build-up to hit the evaporator fan. Repair the defrost system to eliminate the excessive frost.
A faulty compressor can cause a loud knocking or whining noise. Have a service technician examine the compressor if it's making a loud noise. A service technician will need to replace the compressor if it fails because the repair involves recovery and recharge of the refrigerant.
A small toy or other foreign object under the freezer can interfere with the condenser fan. Check for an item underneath the freezer when you hear a buzzing noise coming from the bottom of the freezer.