I have searched and haven't found any information about this noise.
My Subaru started making noise the other day. It is a 2002 Subaru wrx wagon. Sound is coming from the center differential area. Car is dead quiet at start up and is quiet for the first about 10 miles of driving, then sound gradually comes in. Sound increases with speed and is NOT relevant to being in gear or the car being running or not (I can turn the car off and coast and the noise still persists). I have replaced the 3 bearings in the center differential and the sound still persists. I have attached a link to a YouTube video of the noise so if any of the Subaru gurus could listen to it and help me out that would be great. Thanks.
Link to YouTube---> https://www.youtube.com/watch?v=cH1FAovHKBY