Trans case is the only change. Had a reid case and now an ATI . Sqeals in park and in reverse until I push the trans brake button,then it goes away. I use a sprayer at work that sounds like this a little when you release the handle on the gun and the pressure goes through the relief valve spring. Could it be a relief spring in the pump?
Could have a valve body design that only uses the booster in gear and they will make a racket in park or neutral. The other difference to park and neutral is the planetary is running where it is not when in gear so it could be a planetary noise. There is no relief spring in the pump so that's not an option.