Bringing back the "Strange Noise" thread

Hey all, writing this post because Ive been experiencing a strange hum of sorts while driving my 2005 WRX for the past month or so. Some details and background info regarding this phenomena:
- Car is a 2005 WRX, 104K miles
- Noise can be described as a hum (not like a a bad bearing, high pitched noise but more like a lower pitched waaa waaa almost reminiscent of gear noise that can be heard when downshifting and allowing the transmission to slow the car down.
-First noticed it occurring at speeds of about 35 to 40 mph, has recently become more noticeable especially while driving with the windows up.
-Also becomes pronounced when taking a right hand corner.
-Can be heard even when the car is brought up to speed, taken out of gear and is allowed to coast (with Clutch enagaged)
I have dedicated some time to researching this issue on the internet and Ive covered topics ranging from the center differential, to the front half shafts (Replaced OEM front drive axels with aftermarket Driveshaft shop OEM repros last summer due to a torn cv boot and they have been awesome so I dont believe this is the issue), to driveshaft u-joints or even tranny input bearing. I will say there is no feel of vibration in the driveline when the vehicle is under load, which leads me to believe it is not the driveshaft or U-joints. I recently was under the car and did notice a fair amount of grease/sludge build up on the back of the transmission case/centerdiff area where the driveshaft input shaft marries to the center diff. I am not sure if this is pure coincidence or if it is in fact the result of a failing bearing or seal in the back of the transmission case. I can say with a great deal of certainty that the noise I am trying to diagnose originates from the front area of the vehicle, specifically in the transmission/center differential area. I can see that this is a somewhat common issue in older generation WRX's and is relatively difficult to isolate and diagnose. It is also worth noting that I changed the gear oil in the rear diff (after I noticed the noise) and the fluid appeared to show normal wear, no signs of metal floating around in the old fluid. Anyone with a similar experience out there? This car is my daily driver and im pretty in tune with how it behaves and the normal sounds it makes. Thanks in advance for any help/info

- Rob