Samsung RS21DGRS Refrigerator?

I have a Samsung side-by-side fridge freezer, bought new, that has run without problem for the last 3 years. The circulating fan on the fridge side started to make a noise as if it was hitting something. I carefully dismantled the inside of the fridge to find that it was frosted up and the fan was hitting the ice. I defrosted the whole thing then after reassembling it I started it up again. Three months later it has started to do the same thing. What can be wrong and why does it now keep freezing up, how can I rectify it? The temperature is set at -18c on the freezer side and + 4c on the fridge side. Everything else appears to be working ok including the icemaker.