89Legacy

Hi everyone I think I have a noisy diff, Its an 89 Legacy 5 speed manual 1800cc. I'm getting a whirring sound from the rear that gets louder the faster I go and to the point where the stereo doesn't make the drive acceptible however I don't like BOOM BOOM DUFF DUFF too much . I have a new clutch and drive shafts and the wheel bearings are good as far as I know. Any advice would be good as to where I should start. Also when I engage the AWD button on the stick when on the road it feels like the back just winds up and I cant go back or forward. Is it because i'm on the tarmac or is it buggered. Thanks for reading and hope you can help.