What is the loud banging noise when the drum rotates on my White Knight Gas?

My white knight high speed gas tumble dryer has suddenly started making a loud banging noise on rotation. Yet still works if you can put up with the noise. It sounds as though a brick or some heavy metal object is being carried round part way until it comes to a stop and falls back. I have taken part of the casing off and looked at as much as i can, but there is no sign of any loose object. Could it be the bearing? If so is it worth repairing?