I have a cobot that has been deployed that suddenly developed a tick while moving the base joint only. Polyscope has not recently been updated and no changes have been made. It does not sound like a bearing as the tick is consistent with rotation could the brake mechanism cause this? There have been no faults as of yet.
The brake is most likely the culprit, yes. I have seen it several times before. 
It’s not possible to buy a new brake from UR, though. They only offer a repair of the whole joint. But you can contact your local UR distributor for more information.