Unexplained droning

[Tank.]

So almost at the end of my tether!

I've had a droning at the rear offside of the car for a month or so now. The noise is only really noticeable between 20-40mph that's when it sounds at its worst. Doesn't get worse above that speed but still slightly audible.

After several people saying I was a wheel bearing I forked out and got that done.
Got the car back two days ago and guess what, still doing it.
Yesterday in a hope it was tyres rotated rear to front and vice versa.
Nope still there.

I'm dreading it being the rear diff.
Can sound travel from the centre to the wheel for it to appear it's coming from there?
Has anyone had to replace the diff and at what cost?
Could the CV make that kind of noise?
It's a 3.0 tdi Quattro btw

Many thanks
Dave

 

[Tank.]

Anyone want to share their thoughts?

I've done a little more investigation to try and get to the bottom of the issue.
Did a 45-50 mile drive yesterday at steady speed, got home and the rear diff was burning hot to touch. Very little experience with diffs so don't know if this is to be expected.

Also with the rear end off the ground on axle stands, car out of gear and handbrake off, spinning one wheel the opposite wheel doesn't turn the centre prop shaft does is this a problem or normal behaviour?

Has anyone had to replace the diff on their Quattro?

 

[c4a5er]

I would expect the diff to be warm externally even hot in these current ambient temps. So it depends on your idea of 'burning hot'.

The behaviour on turning one wheel is normal....if you could hold the prop still the other wheel would turn instead

Usually an issue with the diff would produce noise centrally so it's odd that you can hear to one side. Maybe changing the diff oil would give some indication, either from condition of old oil, silver appearance, or change in noise level?

 

[Tank.]

Think next task is to drop the oil and check the condition.
Hot is not being able to hold my palm against the diff for more than a couple seconds before it becomes unbearable.

With the front windows down its also possible to hear a constant scraping/friction sound, almost like a stone stuck in a caliper but that not being the case

 

[c4a5er]

Further thoughts..... I may have been too hasty saying the behaviour turning one wheel being normal. It is for rear diff behaviour but thinking more, If yours is permanent 4x4 the prop shouldn't turn unless front wheel up also?

 

[Tank.]

Even if the car was in neutral rather than park?

My concern was more that I thought the opposite side should be spinning rather than the propshaft.

Had wanted to get it into a garage today but it looks more like maybe tomorrow now or Monday since I'm currently on nights

 

[c4a5er]

With permanent 4x4 cars as far as I know, in the situation you describe, the propshaft should be held still by the fact it's permanently connected to the front drive. So I would of thought when you turn one rear wheel the other would turn. (in the opposite direction)

Alternatively if you jack up one side of a 4x4, turning the rear wheel would make the prop and front wheel turn.

This is all providing the car is in neutral and handbrake off. If you put it in park then the propshaft is locked.

 

[Tank.]

So unexplained droning is now explained.

Rear diff is "terminal" according to specialist.

Managed to source a factory reject part. Not fitted to car, done 0 Kms. £350

Much better than the £3.5k for a brand new part and with this part having a 12 month guarantee its well worth the risk