Subaru Impreza 2.0 GX 2002, strange whirring noise
 

Hi all, my name is Mike and I have stumbled upon an issue. I could really do with some fellow subaru owners help, so a little about the car first of all.

I bought it yesterday as a reliable (hopefully...) cheap run around for my brother. Its a 2003 Subaru Impreza 2.0 GX Sport wagon, lowish mileage of 85k miles, recent cambelt, plenty of services etc. Overall it seemed like a good buy.

I noticed on the rear differential at the time there were signs of what looked like an old oil leak, coming from above it, so I assumed after a little research its likely to be the breather pipe that can sometimes split.

Anyway, it was bought in a city centre so I couldn't really get it up to much speed on the test drive but all the gears felt fine on the test drive I did manage to do, hence missing the next issue, when the car gets up to 40mph, the slightest amount of throttle will initiate a audible whirring noise, almost like a whine, it continues to happen until 50mph where it stops. Without any throttle it is inaudible, the tiniest dab of throttle and it reappears. It happens irregardless of which gear it is in, it's the speed that brings this noise out.

Some googling suggests the central diff bearings could be at fault? How likely does this sound? Is it possible it's the rear diff but the sound is presenting itself upfront? I might change the rear diff oil, as it looks a very easy job to do on the driveway at the weekend. But if it's the front or central diff, they share fluid with the transmission correct?

Are there any other suggestions on what could be causing this noise?

Any help would be greatly appreciated.

Thanks

Mike