Grounding sound in transmission

I am back with a yet another request for advice...
As you know, we have now cold weather in Ontario (Canada).
When I start the car after hours of leaving it in the cold, I hear from automatic transnmission - (Biuck Century 2000), some rumbling sound coming out from transmission, (when the gear shift is in 'P' or 'N' position).
When I shift to 'D' 'R' and the other selections, the noise stops immidiately.
Is this normal? Is it because the engine is cold? Or because the automatic tranmission fluid did not have yet a chance to warm up?
Should I worry about it?
BTW, I like this forum, wonder why more Buick owners do not participate in it? Strange, we have here experts willing to help and as it is, they do not have much work. (:-)
Could it be that Buicks do not have any problems worth to write here about? (:-)
Regards
Peter Solarik

It is possible that this trans could benefit from a servicing. I hesitate to recommend servicing a trans that has not received regular service as it tends to create problems. There is the possibility that the sound is the fluid "whistling" through a restriction in the filter. When the engine is loaded, the idle drops, reducing the pressure. Though it is also possible for the torque convertor to create this problem as well, I doubt it. It is also possible that the lower temps are thickening the fluid enough to cause the noise. Does the same problem occur warm? Ray

It is possible that this trans could benefit from a servicing. I hesitate to recommend servicing a trans that has not received regular service as it tends to create problems. There is the possibility that the sound is the fluid "whistling" through a restriction in the filter. When the engine is loaded, the idle drops, reducing the pressure. Though it is also possible for the torque convertor to create this problem as well, I doubt it. It is also possible that the lower temps are thickening the fluid enough to cause the noise. Does the same problem occur warm? Ray

Thank you Barrett!
I will get the car warm enough on the highway and listen whether the noise will persist.
Thank you also on your input regarding the Black Box controversy.
You and Flatrater are really a big benefit to this forum !!!

I have the 2000 century too. But I don't have your noise in that car. But I hear the same type noise in my 1999 Dodge van. But only in reverse and only when initially starting out cold in the morning. In a minute or so its gone - What, me worry? But I live in Sunny California where its never that cold. I'd have it serviced if you haven't had that done. Then they could look for clues like metal fragments in the pan. Maybe I should have my Dodge serviced too.
--Paul