Gearbox whine

I asked this question on another forum, but the respondents all went wierd and got off on a tangent about Steve Irwin and whether Toyotas are better than Nissans - anyway hope I can get a sensible response here.

One week ago I had the 5th gear in my GU Patrol replaced due to
spline failure. Since then I have noticed that in the mornings (cold
at the moment) when I first take off, that there is a high pitched
whine that appears when I back off on the accelerator. This high
pitched whine lasts for about 5 to 10 minutes until I guess all the
parts warm up.

If anyone has experienced this noise after work on the gearbox, my
questions are:

1. Is this going to be a problem that I need the dealer to look at?
2. Will it go away as the gearbox "settles" with its new parts?
3. Why does it go away after about 5-10 minutes?

I have burnt my bridge to a certain extent with the dealer, as it
took 5 days to fix the gearbox after I was assured it would only take
two. So I do not really want to go back if this is not going to be a
problem and the noise will simply go away.

Any information would be greatly appreciated.

Peter