The evaporator fan is at the back of the freezer, behind a plastic panel. Unplug the refrigerator to defrost the freezer. Built-up ice may prevent the removal of the back panel. Remove all frozen goods from the unit and let the ice melt. Once melted, remove the plastic plugs from the panel. Remove the screws holding the panel in place and take out the panel.
The fan is at the back of the fridge. Some fans have a cover that needs to be removed. Take out the screw holding the cover in place. The fan should be exposed. Remove the screws holding the fan in place and pull it out. The fan will still be connected to wires, so don't pull too hard. Remove the wires by tugging on the connectors with a pair of needle-nose pliers.
Contacting the fridge manufacturer is the easiest way to find a replacement for the fan. A model number should be printed on the fan itself. If not, use the owner manual to find the part number. If this fails, a call to the customer service department of the manufacturer should solve the issue.
Attach the wires to the wire terminals on the new fan and put the fan back in place. Insert the screws and tighten. New screws may come with the fan kit; use those to prevent breaking the fan's plastic. Replace the cover if applicable and replace the panel. Plug in the fridge and allow it to run for a while to hear whether the fan kicks in. If you hear loud noise again, the new fan may also be faulty.