Droning noise at speed

[Father Ted]

Hi guys. (Sorry for the long post but I might as well give all of the details).
After a few years of fairly trouble free motoring, just normal age related niggles, I now have a problem which I can't identify. So I hope some of the experts on here can point me in the right direction.
Once my speed gets to about 40mph I can start to hear a droning noise. After about 50mph it becomes quite loud.
I've had this before when O/S front wheel bearing was failing.

I couldn't place where the noise was coming from (seems to be all around me), but my son confirmed that he could hear it mostly from front left and could feel a vibration through his foot.
I had previously noticed the plastic backing on N/S front bearing was coming off (I was changing ABS reluctor ring), so presumed that this bearing had run dry. I had a bearing ready to fit, so fitted it yesterday.
This doesn't appear to have made much difference.

I don't like swapping things out until I find the right bit, so can anybody suggest how I can narrow this down? Clearly I can't put my head under the car at 50mph.
What else could cause a speed related droning noise (I have confirmed it is not related to revs by dropping into neutral) and how do I best rule one in and another out? Would there be any secondary symptoms I could rely on to say it can't be this, or must be that?
Transfer box?
Front Differential?
Rear Differential? (Presuming my son is wrong about it being at the front).
Prop shaft? (Would I feel a vibration?)
Drive shafts? (Again, would there be vibration).

And, I suppose, how easy are these components to change? I'm really hoping it isn't transfer box. (Prop is still locked to gearbox, so spline hasn't failed).

[Father Ted]

A little more information after reading more posts on here.
I have tested for spline failure by grabbing the prop shaft and trying to turn it.
This is what I was advised before buying the car.
I can turn it a few degrees (just enough to know it turned) before it locks.
Should there be any movement? It has always been like this and the noise is new, so I presume they are unrelated, but......

[gmonag]

If it is a wheel bearing, the noise will normally change as you corner, changing the load on the bearing.
Otherwise I would suspect tyres.

[Father Ted]

Thanks.
I noticed on Monday that the noise goes when cornering hard left. (But it does have to be hard cornering).
I presume this means N/S rear as the N/S unloads and the front is new.
Hopefully rear is just as simple to replace as front.

[Tannaton]

Originally Posted by Father Ted

Thanks.
I noticed on Monday that the noise goes when cornering hard left. (But it does have to be hard cornering).
I presume this means N/S rear as the N/S unloads and the front is new.
Hopefully rear is just as simple to replace as front.

If anything the rears are easier… you needs a few sockets and wobble bars to get all the bolts out though.

Don’t buy a cheap bearing on eBay, go genuine or something like SKF etc, if you’re on a budget then buy genuine second hand….

[60041]

My XC90 was very sensitive to worn tyres, especially Pirellis, so it might be worth trying it with the spare on the N/S/F and seeing if it makes a difference.
I concur with those who have advised against cheap or unbranded bearings, I have heard of them failing after only a few hundred miles, so always buy known brands; SKF or FAG can be bought from GSF for just over £100 and are every bit as good as the OEM parts.
In my experience, the XC90 is prone to bearing problems; I ended up replacing 3 on my car before it had done 70,000 miles.

[Father Ted]

Thanks All.
Well, I went with the Ockham's Razor principle. As everything pointed to the bearing and as these are known to fail (after 20 years and 130000 miles I can't complain), I replaced the rear bearing.
Noise has gone.
Rear is much easier to change than front and took a little over two hours.
One tip I found is this. The videos online show to unbolt the lower control arm to get to the bottom bolts.
Reading the Haynes manual it suggests to jack the wheel instead. This worked a treat. I did need a long extension on my socket, but didn't need to remove any suspension components, just the brakes and disk to access the bearing.