SR-S2028 Aweful noises.

I have this american style fridge/freezer and its been fine since i bought it (second hand) but i woke this morning to find it making a loud noise, kind of like a lawnmower.
When i opened the fridge door it stoped.
I closed the door again and it started up again.
I pulled it forward and the noise has stoped, and stayed quiet for the last half hour.
It sounds like it might be ice building up on the blades, is it going to be worth turning it off and defrost it, or should i get some1 to come and have a look at it before it goes ‘ka-put’?
the temps are fine aswell.

Thanks.

Tom