Couple questions...
Does the noise go away when everything is warmed up?
Does it make any noise when idling in park or neutral?
Any other operational issues to note?
How does the fluid look and is it at the correct hot-running-park level?
Sometimes, in older trannies, you'll get growling noises from the converter or pump; occasionally from the forward clutch/reverse-high clutch frictions.
Sometimes, too, worn engine/trans mounts can impart some funny noises which may seem like they're coming from elsewhere but this shouldn't be temperature or gear dependent..
Lastly, if you're getting some shuddering going into gear, the clutch seals might be a bit worn and aren't sealing well when cold, preventing deliberate engagement.
If the fluid looks OK and the trans operates fine otherwise, I would tend not to be concerned...just make a mental note of the condition and be aware of any changes which might occur in the future.