Is your bike a '75 or earlier? That sound is what's called clutch rattle. You're hearing the plates rattling against the basket. When you pull in the clutch,or disengage it, it goes ''limp'' and rattles
less. Honda made some changes in '75 that helped reduce the rattle,but it never really went away. Nature of the beast. The pre-'76 bikes will make more noise (so I'm told) than the ones with the updated parts,but they all rattle. I've grown to love that sound myself.