Just purchased a 1982 CB750SC, 26k miles. Making my punch list of items that need addressing. Bike idles rough but runs strong. Valve clear check 1st on my list. I did notice that at idle, there is a knocking sound coming from the engine that is reduced when I pull in the clutch or when engine speed us increased. Clutch does not slip, can spin the rear tire and functions best I can tell thru all gears. Any thoughts?