Remove the brush assembly and check both ends for wear. Also check the 'bearings' in either end of the brush head. If you can rattle either end by hand when it's fitted then the bearing is worn causing your issue. Also check all the brushes are there and not missing.
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After looking online it looks like there might not be parts to fix it apart from the obvious. Hopefully I'm wrong.
Remove the brush assembly and check both ends for wear. Also check the 'bearings' in either end of the brush head. If you can rattle either end by hand when it's fitted then the bearing is worn causing your issue. Also check all the brushes are there and not missing.