I don’t see the fix being a series of transmission flushes. If the transmission needs to be serviced, have a proper service on it done, dropping the pan, cleaning/changing the filter, new fluid. You could repeat that 3 times over the course of a month I guess, in order to change out most of the fluid. There’s so manhy things that can cause rattling, it’s all but impossible to speculate. One thing that comes to mind though is to have the engine mounts checked for abnormal wear.