I think i am having the same rattle you are talking about. My tractor makes a rattle sound which I was contributing to the transmission. I have an intermittent rattle when in gear until i push on the clutch or put the lever into neutral. Sometimes it worse than others and sometimes it goes away completely. It is usually worse in low than high. Is this similar to the rattle you are talking about. Id really like to get mine fixed if its something that going to do more damage, talking to the dealer over the phone he thought it might be the transmission flex plate.