Whining bevel gear? Should I worry?

[PVSPete]

I've had a 2005 XC90 D5 auto 135000 miles for a few months and have noticed a background whining noise.
Its not bad and many passengers can't even hear it but I'm concerned that it could be the start of a problem.
It can be heard most when cruising at 30 to 50mph and gets no worse when accelerating. It disappears when you take your foot off the accelerator pedal. The tone changes a little with speed but not much.
I don't think its a wheel bearing as turning one way or the other has no effect.
I'm sure its not coming from the back.
So I sucked out the front bevel gear oil - it was brown. Yes brown, like gravy.....had the previous owner put an additive in there or is it rust? I refilled it with fresh oil to the level plug (which I now know was wrong) drove it and think it was a lot better. A week later I changed the oil again which was still dirty. This time I refilled to the level plug and then removed 100ml as instructed by VIDA. The noise is still there and may be worse again.
I'll change the oil until its clean but don't think the noise is going to go completely.
So my question is - should I worry? When these bevel gears are in good condition are they totally quiet? Or is a light whining normal?
PS. the AWD seems to work so no issue with the splines.

[ca2n]

Watching this thread with interest as my 2004 XC90 D5 (183,000 miles) exhibits a very similar issue and I also share the same concern.

[gmonag]

I'm sure its not coming from the back.

I would not be so sure. It is very difficult to place a noise like this.

My car whines when under light load. I suspect a gearbox bearing or diff, but it has not changed in 5 years so I turn up the radio and ignore it.

[john langrick]

Hi, my XC90 my 2003 d5 geartronic has always had a slight whine from the area of transfer box. The car has 170k and in my ownership has 30k of these miles.

My spline gear had failed before I had the car and I thought fixing this would resolve. In the end I had to replace both spline gear and transfer box as the stripped spline gear had worn the input to transfer box. Both oil in transfer box and transmission changed, but I can still hear that slight whine.

If is not loud and really not noticeable unless radio off and a particular smooth road at steady speed.

To cap it all due to a cracked head, I had the engine and mounts replaced... I asked my indie to have a good check around and other than the turbo (which had worn bearings and replaced,) all ok.

And I still have same slight whine.

Btw, tyres all changed twice, still a slight whine which I really now ignore.

John l

[PVSPete]

Thanks for the reply. I was thinking of buying a used bevel gear / transfer box but my concern is that I'll do a load of work to be in the same place . . . I did find a company online who rebuild these boxes for around £550 exchange. They specialise in Freelander 2 running gear. Their website info suggests uneven tyre wear or a mix of different tyres is the cause of the box failing. Another Swedish forum showed a strip down and suggested lubrication was the problem. Apparently the pinion bearing (output) is lubricated by splashes of oil finding their way along an oilway. Oil grade and level are critical to ensure good splashing...!