Whining - front or rear?

[PVSPete]

I've posted before regarding a whining noise from the drive train of my 2005 D5 XC90 auto. 140,000 miles and a few people said they'd got similar. The noise isn't there for the first few miles from cold, but then starts and is most noticable around 40mph on a light throttle. When the throttle is backed off, the noise stops. Open the throttle a little, it comes back. I've recently found a post from cpu_man who describes just this. His was ultimately put down to a failed pinion bearing in the rear differential.
I had previously found that the front angle gear / bevel gear unit had very rusty looking oil in it and despite changing the oil 3 times I felt it was probably the source of my noise . . . so I located a replacement used angle angle assembly from a crashed car and last week fitted it. The noise may be less pronounced, but is still there.
So I'm back to ask what you think?
Is the secondhand front angle gear a bad one?
Do I have a failing rear diff? Or Haldex?
Is there a test to help decide where the fault is?
Is there a wireless microphone you can connect to a phone app to help locate the source of the noise?
Any thoughts appreciated.

[Moose Test]

Originally Posted by PVSPete

Is there a wireless microphone you can connect to a phone app to help locate the source of the noise?

There are such devices but they aren't necessarily inexpensive, nor readily available. This UK site lists a few of them, including a more conventional radio version. https://gb-jbtools.glopalstore.com/s...ic-device-kit/

Still not 'cheap' but close to the price of a visit to a Volvo Dealership, who hold such equipment (I had my rear diff diagnosed in that way, to definitively differentiate it from the Haldex) and you could always recoup something by selling it on after you've made use of it.

[PVSPete]

Thanks. I've seen "chassis ears" popular in USA but expensive. There is a DIY version using lapel microphones, extension wires and a laptop recording to an audio program which I could try.
In the meantime I've just removed the propshaft and shall drive home shortly. What I don't understand about this rear diff test is that the propshaft is disconnected from the rear diff by the Haldex in normal driving as I understand, so should have no influence on the rear diff . . . I'll report back.

[Moose Test]

Originally Posted by PVSPete

What I don't understand about this rear diff test is that the propshaft is disconnected from the rear diff by the Haldex in normal driving as I understand

I think I recall reading that in the case of Volvo's Haldex implementation, at least in the XC90, the rear wheels always receive a minimum of something in the order of 5 to 10% of the available output.

[PVSPete]

Rear propshaft disconnected and removed . . . but no change. I still have my road speed whine which disappears when I take my foot off the throttle. I'm expanding my thoughts to include alternator, air con compressor, maybe a vacuum leak? However these things would be engine speed related and would probably change tone as the car changed gear. That said, the engine speed drops immediately when you take your foot off the throttle unless the gearbox is locked up...... I believe if I use the auto box in manual mode I can make it lock up at 30mph in 3rd and 40mph in 4th so I'll try that and see if the noise changes. My mate can't see the problem and thinks I'm hearing things . . . but he does drive a Series 2 Land Rover!