plungefrog

Greetings all, My wife's 1997 Impreza 2.0 Sport has developed a nasty noise, which I suspect is the rear diff but I'm not sure. The noise started a month or so ago and has got noticeably worse since. It is a moaning/whining, a bit like an old bus with a knackered gear box. It sounds like it is coming from under the rear end of the car. The volume of the noise increases with road speed; it becomes noticeable at about 15mph and gets louder until around 45mph when it's really very loud, the noise then stays about the same volume at higher speeds. Lifting off the throttle, dipping the clutch or putting the car in neutral makes no difference -the noise changes only with road speed, not engine speed. I would suspect a wheel bearing but going straight ahead or turning makes no difference to the noise. When I get two minutes I will change the rear diff oil but I wonder if the diff is broken, or if it's something else. Any thoughts please? thanks, Tim