I have the same noise. It's not the CCT or Valves on my bike. It was due up for a major service so the valves got checked and CCT checked. None of that fixed the problem. Mechanic also torqued down the header and checked some other stuff. After some research on ninja250.org I think the problem is this:
http://faq.ninja250.org/wiki/What%27..._the_clutch%3F