In Park. At least backwash filter gently. Whine from the pump is caused by fluid starvation. Problem is that clutch material is hygroscopic. It will absorb water and displace fluid. Water will react with steel plate of the clutches and cause delamination. I think that is what you see in the pan.I added 5L and planning to use the coolant line hose draining method with 6 additional new liters I still have, I am thinking if the pump is bad than in theory I should not have a flow of fluid pumping out correct?
Also what gear should I do this at I've seen people online do it with "Park" and also some with "Neutral" and thoughts.