Transmission whine at 40-50km/h

There is a whining noise related to vehicle speed and is most noticeable under the lightest acceleration at 40-50km/h, eg keeping a constant speed on the flat. It basically goes away on harder acceleration and is not there when decelerating. All other noise is as expected and not intrusive or unusual. Car is 2005 with 212k kms.

I changed the ATS fluid recently and didn't notice any metal "glitter" in the old fluid or much stuff on the magnet - it was just a mm of black sludgey stuff that didn't seem metallic. The fluid that came out was dark as I assume it hasn't been changed before - no chunky globs or anything that would spell trouble..

Any ideas what is causing this? The car is great other than this annoying noise which happens to be at speeds you normally do around town...

 

It is definitely transmission related since it increases in pitch with vehicle speed and also "tremelos" when you get a judder through the trans, eg. ICE turning off/on or changes in acceleration due to bumps.