Noisy Diff?

[alexfwalker]

I'm getting a grinding/scoring type noise from my X1 (xDrive Auto 25d, 90k) which I had initially thought was brakes. However, I have just had to have discs and pads changed all round and was surprised to hear that the noise was still there.

Started to take it a bit more seriously and have done a bit of research.

I am starting to conclude (regrettably) that the noise is actually the diff. Listening to it more carefully now that I know it's not brakes, it sounds more like a shot wheel bearing. However, I can 'play' it by applying throttle. If I add even a tiny bit of throttle, the noise disappears. If the car is coasting (foot off the brake) then the noise reappears. Speed doesn't matter, so I'm ruling out wheel bearings, which I think confirms that it's the diff.

Does that train of thought make sense and are there any more tests that I can do to confirm? Assuming that it is the diff, how much does this sort of thing cost, and what are the consequences of leaving it - in other words, is it just an annoying noise, or is the diff eventually going to give up entirely?