Any ideas whats wrong with my fridge?

Its bout 4 years old . This evening I noticed a kindof buzz sound then a click . It did this intermittently all evening and was coming from the bottom rear of the unit. Just now I got up cause I heard the fridge beeping like when the door is left open too long although the door was shut . Sure enough inside the freezer stuff is thawing and it doesnt seem as cold as it should it the normal section. Give it to me straight - does it sould like an expensive fix?