Miele T490 tumble dryer clicking noise?

My Miele T490 dryer makes a once per drum rev clicking noise. On investigation there is a 'rocker' arm with brushes to the right side of the drum. Two sets of 'brushes' mounted on the arm contact the outer surface of the drum. the click is caused by a clip and bolt head which passes the arm every rev and hits the arm. What is the arm assembly for? Is the bolt head making contact because the brushes are worn? Do I replace brushes or look elsewhere?