Help - Trouble with Bosch Rotak 34 Lawnmower

Could really do with some help trying to fix my Bosch Rotak 34. It's been a great lawnmower but recently it has started to make a very loud noise which obviously isn't right. It starts off OK, as can be seen in the video but at about the 10 second mark, the internal cover lifts up and you can see sparks where the motor brushes are. The brushes themselves only look about half worn so I'm not sure if they are the cause. I've taken everything apart but can't see anything missing. Only thing is perhaps something to do with the fan near the bottom end of the motor. I think this is causing the internal box to lift. I can't help feeling there is a simple fix. If anyone has any suggestions then they would be gratefully received. Thanks.