If the noise stops when you open the door then the fault will be the fan inside the freezer at the top.
Its more then likely got ice forming around the fan so the fan blades hit the ice and causes the noise So as soon as you open the door the fan stops so thats why the noise stops.
What you need to do is either empty the freezer and do a full defrost. Turn it off for 24 hours and keep the freezer door open. This will allow the ice to melt in there.
Or empty the freezer and from there you need to remove the plastic housing at the top of the freezer and behind all that you need to melt the ice in there. This way is a bit more difficult as if theres quite a bit of ice it makes it harder to remove the panel.
So the best way will be a full defrost and that should cure the fault from there for you.