I read on here that some transmissions had a rattle so it's recommended to use 15W40 motor oil instead of trans fluid. You might give that a try.
Also check your engine's timing chain for excess wear. Sometimes a worn timing chain can contact the timing cover at certain RPM's making a rattle-like noise.
Re-check the damper plate too. I believe there is a newer better design available over stock?
I was njskier on here.