In reply to nufkin:
It would seem that when I close the freezer door, the little fan starts and it rattles, seemingly harmlessly perhaps, and it's the vibrations being magnified through the whole fridge/freezer which makes it loud and serious sounding.
I'm going to look into getting the little fan and/or motor replaced and see what happens, it isn't the compressor at the bottom at the back, but the little fan which draws air from the freezer unit. Leaving to freezer door ajar stops it making a noise for the time being (there wasn't much in it anyway).