904 loud whining in gear. Help!

two guesses;
as already mentioned is the convertor spline. and second is a planetary stripped out. That happened to me when I installed an A998 gearset on an A904 output shaft. Apparently you cannot do that because altho you can physically assemble them, the splines were mismatched, and I got about 15 miles from home when
zing zing,
sounded sorta like yours does.
In my case, I was walking.