Never experienced that sir. All i can observe is my APV is slow and getting slower..

When i floor the gas, acceleration dont pickup that fast from 80 to 100, then i got some delayed shifts thou i can still hit 130-140kph top. FC improved from 9 to 11 KPL highway after i changed O2S
The humming sound of my front bearing goes with speed. Yours might be from engine and driveline or vibrating parts like unsecured seats. Whats the color of the old ATF, were you able to recover the fluids?
Before going to the big job of A/T overhaul, might want to inspect first the maintenance parts.. Check the driveline from engine supports, propeller shaft down to the axle and rear wheel bearings coz engine load also affects how they behave.