This morning as I rode my bike to school I found something gets wrong with it. The back wheel emits a clicking sound at an interval of several seconds when moving forward. I think either the back axle must have worn down or the ball bearings around have been out of place. This axle may be the second substitute for the original one. It has served for a long time carrying me on an uphill or rugged road. Tomorrow happens to be a weekend and I'll manage to have it replaced if it is really for the matter I have concluded.