It's pretty simple and not to complicated, and for the life of me I still can't understand why seasoned motorcycle mechanics can't figure this one out. You pay good money to send your bike into a shop, they charge you and hand it back to you with the same issue. Much of this, if not all is right in the poor design of the rocker box. There are two corrections that can be made, depending on which one is causing the noise. The problem is very visible and the cure is not complicated. Once fixed, you can't even hear if the engine is running or not.
Worn valve guides don't make noise and this might come to a surprise, but there is no power loss from loose guides either. Metal to metal makes noise.