It's most likely that the bearings are loose; Remove the wheel and check for play in the axle, if so tighten the cups as in any normal hub. I found this very comprehensive if you need to go deeper; http://www.bicyclehub.co.uk/index.php?option=com_content&view=article&id=31&Itemid=105
@Redmist started
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It's most likely that the bearings are loose; Remove the wheel and check for play in the axle, if so tighten the cups as in any normal hub. I found this very comprehensive if you need to go deeper; http://www.bicyclehub.co.uk/index.php?option=com_content&view=article&id=31&Itemid=105