The sound of the dryer in the laundry room

Like a heartbeat, the drum revolves: wuh-WUH ... wuh-WUH ... wuh-WUH. The buzz of an electrical appliance. A scratching like stiff hairs being dragged along a sheet of metal. A tumbling of solids like rocks rolling down a set of stairs. But I only put towels in this load. What could be tumbling?