My 300D has a 722.3 and it made a whirring, howling noise in reverse. It was pretty slow engaging into reverse and occationally I experienced the noise in reverse when accelerating some then letting off the accelerator. The noise occurred during the coasting in reverse....
I had poor shifting in forward gears 2-3-4, but no leaks. The tranny failed finally and I installed a reman. unit. The funny part is the thing worked fine when I bought the car, but the car had been sitting a while. After using it a while the problems surfaced... it was all down hill. The replacement is working nicely. I am looking forward to someones technical answer to your issue.....

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GS (tnarg)

'67 200D, 225,000 mi - "FINTAIL", 4 on the floor and loving it!
'83 300D-T, 229,000 mi
'85 300D-T, 234,000 mi
'86 524td BMW, 230,000 mi; In-line 6 cyl. Turbo Diesel !! (BUT I SOLD IT AFTER PUTTING OVER 200k ON)