Was the transmission fluid low then? How much did you need to add? Usually when an automatic transmission starts to slip (from wear or being low of fluid) there’s no loud rattling noise. But if the transmission is driven some time while low of fluid internal damage could occur and produce rattling noises. More likely the rattling noise is coming from one of these
- loose exhaust component
- damaged torque converter or flex plate
- faulty axle/cv joint
Most any good shop should be able to put it on their lift and figure out what’s causing this without much effort. Suggest to go with that approach. You could get lucky and find the problem is minor.