This is probably a defrost issue. Have you got frost on the back wall of the freezer? Remove the panel and inspect the evaporation compartment for ice. Look for a swollen or failed defrost termination thermostat (clik pik to enlarge) This dude shuts down the heater element during defrost cycle which last for 21 minutes. The heater only stays on till 50f is reached inside the evap compartment. Then the frost is supposed to melt during the 21 minutes. If the heater fails to come on due to being bad or having an open defrost heater termination thermostat then the defrost will not be done and ice will build, fan will start to howl as ice builds up near it, etc. Warm temps in the fresh side and frost in the freezer results. If both the terminator and heater are good, then the defrost timer is not advancing or the newer defrost control has died. The termination thermostat must be 25f or less to check it for a closed condition. At room temp it will be open.