Well, there's not a lot of possibilities in a case like this. I have a couple theories.
Does it whine like a power steering pump when its low on fluid? If so, could be the front pump having a hard time moving the required amount of fluid. Since in P there is no pressure the volume increases as it is continually dumped right back to the pan as fast as the pump can move it. Do you have full synthetic trans fluid?
If it's more of a metal sound, like a dry bearing spun with air, it could be a weak bearing in the converter or input shaft area. It's about the only other thing that changes between P and N, and charging the converter could load the bearing and quiet it down.
I'd love to hear a clip of the noise. I haven't heard any noise from mine down to -28F, something's not happy.