Hello, This is sounding like a defrost issue. Somehow, the noise created by the fan motor inside the freezer is because the evaporator could have accumulated ice already and the fan blade is touching the ice . That is why the noise would disappear when ever the door is open because the fan stops when you open the door.
but in some cases, ice accumulation could be because the door could have been left ajar or you could have put in food that has a lot of moisture or is wet.
First solution we suggest is to thaw the freezer if you know how to remove the back plate inside the freezer and thaw all the ice with a hair drier.
But if you can not remove the back plate, you can do it slowly if you can remove all the food, transfer all to another unit and unplug the noisy unit for one day with doors open to thaw all the ice.
Then plug it back and observe if it would still make some noise.
If it is displaying -18 and yet the unit is not cooling, can you verify if the compressor at the back is running or not? You may have to remove any panel at the back to expose the compressor. The black steel ball . Be careful, it could be too hot to handle.
Do the test while the ice is melting and yet the display is -18.
If the compressor is not running, then you could have two problem. The least expensive one is the thermistor that indicates the display. It could be defective that is why it is not sending the right signal. But if you replace the thermistor and the same thing is happening, then this could be a problem with the main board controller which is a little bit expensive.