My 2009 Outback has developed a speed related whine from the back somewhere. I've checked the diff oil and that's OK. From conversations on this forum it seems a wheel bearing is a possible culprit.

I'm using the car for 500 miles on holiday next week, so if it is the bearing I'm thinking it will be OK to leave it.

The one thing I did want to make sure of though, is that it isn't caused by the mismatch in tyre sizes between front and rear. I had 2 brand new tyres put on the front about 6 months ago, whereas the backs are about 50% worn. If it is confusing the AWD system would it cause a whine like this?

sounds like it could be a bearing (mebbe buy 1 and take with you in case)

 

i do know it's always best practice to change all 4 tyres at same time, ,50% difference in tread depth is not a good plan imo...i'd bite the bullet and get a matching set

then see if that improves the whine.

I've bought a 09 2.0D about two weeks back. When I got it I knew it needed at least 1 wheel bearing on the real which the garage replaced before collection. However, after I had collected it I noticed another faint whine coming from the rear... we decided to replace the bearing on the other side and that solved the issue. 

Now thought, only a few days later I'm experiencing a very similar noise from the front... Surely not another bearing! But you're issue sounds similar.