I always feel a dill trying to explain a mechanical noise, but here I go again. 1995 CB 250, done 75,000. Seems to run fine and nearly always does what she is told. We have not travelled all that far together so I am still learning her little ways. Just running normally on the road I become conscious of an engine rev related whirring noise. Sort of like that sound you associate with a shaft drive. The chain seems properly adjusted. I like to know what my machines are trying to telling me. Any suggestions?