A failed pump or even a failed sprang. But what I would do first is pull the front cover off the trans, check to make damn sure the converted bolts didn’t come loose and run it in park, see if that makes it sound louder or not. When the converter failed on my charger, removing the front cover made it much louder.
Then I’d drop the pan before pulling the trans. Check obvious , filter etc. if all that checks out, I’d call that Stan guy and see what his thoughts are