I think you are right about it being transmission related. The way you analyzed the problem does not leave much room for any other component to be the source of the noise.
Overdrive is sort of, "separate" from the main transmission shafts. That is for lack of a better word. Overdrive is not considered a gear because of this reason. The transmission input shaft is actually turning faster than the engine crankshaft when it is in overdrive. In Drive when in top gear, it is turning at the same speed, not in all cases, and Overdrive is a separate "Unit" to allow the gearing to sort of, "turn around" and get the transmission turning faster than the crankshaft. Sorry for all the quotes, but it is the best way I can explain it. You can even purchase bolt on Over Drives for vehicles that do not come equipped with one.
So, you might try to service the transmission fluid. Use a clean drain pan. When you empty the pan, look at the last little bit of the fluid in the pan and inspect it under a light for metal. That is a sign that you need to rebuild the transmission or at least have it fixed and flushed out thoroughly.
I think you have made my job easy because you have done what I would normally have to walk people through to find out if the noise is related to something that is related to engine speed or road speed or neither. The hard part of this is agreeing with you in that the transmission is the problem. However, since the transmission is not slipping or shifting differently, you might have caught it early enough to at least lengthen its service life or maybe even have it repaired without rebuilding it.
In any case, try changing the fluid and looking for metal or have a repair shop do the same and ask them to save the fluid for your inspection. If you feel that is necessary.
I hope this helps. Let me know how things go and I will keep an eye on this post for your reply.
Saturday, February 12th, 2011 AT 9:29 PM