If you have the deck baffles in place you can use them as a reference to check for a bent shaft or blade. Pick a spot on the baffle and see how far below that spot the end of the blade is. Then turn the shaft so the other end of the blade is at the same spot and it should look the same. If one end is higher than the other that means either the shaft or the blade is bent. If you don't have the deck baffles in place you can just measure from the end of the blade to the underside of the deck instead of eyeballing against the baffle. Either way, check both ends of the same blade against the same spot on the deck or baffle.
If that reveals nothing then try disconnecting one motor at a time to see if you can isolate the problem to just one of them.
Have the deck motor bearings been replaced?
When you say it rattles and bangs when mowing, is that only when you're moving or does it do that when the deck is down but the tractor is stationary?
-Ben