My first thought was torque converter. Slippage and no lock up kills milage. The discription of the sounds seems to indicate a converter stator just touching the impeller then starting to eat in as the bushings selfdistruct. This sound first shows up when the car is put in gear as resistance is applied to the converter and continues until lockup at about 20mph depending on rate of acceleration which affects shift-up points. Lock up eliminates the speed differance between the stator and impeller thus stopping the noise. All this looseness can reduce pressure to the clutches and bands causing them to slip and of course burn up the tranny. I would be intrested in the color and smell of the fluid.