If the rattle only happens when the deck is right down, check the leading edge of the cutter deck for any loose steel. Sometimes, on early cox decks (with a square top edge), the skirt can break free on the leading edge. If this happens and it's dragging on the ground, it can move into contact with the blades.
Surprisingly, a drive clutch problem can also cause deck vibrations. Do you have to move the pedal forward a long way before the machine drives? If so, send some pics of the clutch system.
