I took my son riding today and after about 2 hrs we started to notice a new noise. Chain was a little loose, so we tightened it but that didn't solve the problem. You can only hear it when the engine/tranny is under load. It's not really loud, but definately noticable. Kind of sounds like sand or grit in a bearing or something like that. Noise is coming from the motor area. What should I check/clean, or what kind of problem might I be looking at here. Other than the noise, everything seems to run and drive just fine.