So my bike has started some thing new. It is making a terrible popping/ crackling noise out of the exhaust at 2000 to 2200 rpms while driving through town barley any throttle. If I let completely off no noise, and if I gas on it harder it doesn't do it either. It also sounds fine at 3000rpms going down the highway like it should. My bike has just rolled over 8000mi would this be the valves needing adjusting? anything will help THANKS!!!