Yep, box has to come out
Torque converter is sandwiched between the box and the flex plate at the back of the engine, inside the bell housing.
If there is gurgling and noise coming from the trans, id suspect that something inside the converter may have let go, might need to take it to a trans specialist if its not throwing any codes.
As RevNev mentioned, there is a service bulletin for that nut at the end of the box, however when I checked my Dec 15 build SV6, it wasn't applicable. I think they sorted it later in 2015 so might not apply to your car if built later than that.