I would suspect that the bearings are going bad. I had a similar problem on my 74 164 4sp OD with the same type of noise you describe. The bearings make noise when the race starts to get rough spots. This can be caused by the fluid level getting low and providing insufficent lubrication to the top bearings. The bearings load is shifted from front race to the back race depending on whether accelerating or deaccelerating. The whine just got worse unitl I had the transmission rebuilt with new bearings. This wasn't cheap as I had it done at the dealer. I didn't feel comfortable going into the OD unit and taking it apart to replace bearings.

91 940SE sedan and wagon