I just replaced the center diff in my girlfriend's 2010 Forester with a 5MT. There were no strange noises before. I was very careful on tear down and reassembly to not mess with the outputshaft setup org to loose the washer on the output shaft. Everything went together great, but on the test drive, there is a loud whine on acceleration that gets louder white the RPMs.

 

Looks like it has to come back apart, but any clues to what I'm looking for?

 

Thanks!!

And answered my own question..... Found the washer that goes between the center diff and the output shaft in the drain pan. All back apart and ready for the second try. Oh well, glad I found the smoking gun!

And answered my own question..... Found the washer that goes between the center diff and the output shaft in the drain pan. All back apart and ready for the second try. Oh well, glad I found the smoking gun!

 

And even better you came back to tell us what the issue was so we can all learn!