When I pedal the bike, it makes a thumping noise and I can feel the thumping on the pedals. I took the shroud off and I don't know what the part is called but it's the biggest of the silver discs, it's not the ones holding the belt. It spins when you pedal and glides when you stop. I think the problem is inside that part but I don't know how to take it apart or if it's possible to take it apart. What do you advise?