727 Trans question.

I just had a 727 trans rebuilt for my Dart Sport. I noticed when the car is first started and put into reverse within a couple of seconds the trans seems to be sluggish. If I put it back into park and let it idle say 30 more seconds it goes into reverse fine. I also here a distinct whine when in reverse. Is this normal? I have another rebuilt trans in my Duster and it does the same thing. Same problem and noise.