I have a 2002 crf 450 that has just recently started making a noise in the engine. The only thing i can compare it to is the sound a skateboard wheel makes when you spin it. I have read about problems with couter balance bearings. If that may be what it is how can i tell. And how hard is it to replace them. Other than that this bike has been great. I have had very little trouble with it since i bought it.