I went to check out a 2006 yz450 today. The bike is in good condition, cranks after 2-3 kicks but when idling it makes a knocking sound like hitting a hammer on steel. The owner says all his friends 450 make the same sound.I don't notice the sound as much when you rev the engine, but could be exhaust noise drowning out sound. The bike seems to have good power. The bike has 71 hours on hour meter. He doesn't know if previous owner had adjusted valves but he hasn't. Is this sound normal or is something inside ready to break? Thanks for any help on what this could be.