Liebherr CNES 666 freezer - loud fan (?) noise

Question

Hi 

I have had my fridge freezer for a couple of years and yesterday all of a sudden it started to defrost with temp going down to -2 (this is the freezer). I am not sure if the door was left open - I don't think it was.
Now it is all frosted and making a very loud noise when door is closed. 
I think it may be the fan. the temp is back to normal.
there is nothing in the instructions about this can anyone help?
thank you in advance
Abi