Are you sure you're not just hearing the rattle of the floating driver? With the clutch disengaged (especially at slow idle) the floating disc will rattle! Antique & early styled tractors did not have the floating drivers so they did not have this problem. As they came up on compression the floating driver would "flywheel" forward taking up all the slack in the splines between them & the fixed driver then when it fires the crank speeds up and IT charges ahead taking up the slack on the other side of the splines. The diesels (70-730) were real bad about this because of their higher compression.
The rattle diminishes as you increase engine speed and flat out goes away when you engage the clutch.
Maybe that helps?