I was going for a cruise today, just accelerating mildly down the mainroad, when suddenly there was a massive crack noise from somewhere around my rear wheel. Very scary sounding. I rode her home gently afterwards, but a bad thrashing noise is evident from somewhere around the rear wheel when you're cruising now. Still rides fine, so its not inhibiting performance or handling. I initially thought perhaps one or more of the rear sprocket teeth had broken, but all the teeth are smooth and unbroken. Rear brake still works as usual. Gearbox is fine. The rear shock has no noticeable changes. What could it be? Sucks, because I'm trying to sell this bike- a couple of hundred $ gone if its a problem.