need to fix samsung rs21wcsv american fridge freezer?

Hi we have a samsung RS21WCSW american fridge freezer, about 8 years old. The fridge part started making a funny noise, which we now know to be the fan hitting ice that has built up. It happened a few weeks ago, and we got rid of ice and eventually it started up (did not come on straight away, my partner not sure what he did but it suddenly started up) and had been fine until today. All iced up again. Have read somewhere that it could be a drain hole that is too small. Any ideas please ?????