how can I inspect the fan on my Leibherr freezer?

I have a working Leibherr 3376-20e, 9987668-05 tall freezer that has started making loud grumbling noises. The noises are intermittent and always stop when I open the door, so I suspect it is to do with the no-frost fan which seems to be housed in a six inch unit at the top of the freezer. I have tried turning the freezer off for 24 hours in case the fan has got iced up but no joy. My question is how do I remove this box to inspect the fan as there doesn't appear to be any obvious screws.