Bit of an update, and cautionary tale to this one.
On Friday the rear diff oil got changed as the howl was expected to be from the rear, and we weren't 100% confident that the oil was correct so the Audi approved diff oil was used.
Yesterday I travelled from Oxford to Dorset and the howl was still present. We racked our brains about it and thought it could be a wheel bearing, so i resigned myself to it getting worse over time but not needing immediate attention (it's a 'payday' problem) and carried on.
We made it there fine, parked up for a few hours and then decided to drive somewhere else in the area.
Less than 5 miles from our last spot I was cruising at around 25mph on a country lane when the car felt like I was in 6th instead of 4th, so I changed to 3rd to keep engine speed up and at that point the car stopped dead as if I had emergency stopped.
No noises or indication of a failure, it just stopped.
I put the car into first, attempted to drive off and it wouldn't move any further forward than 1 foot before it attempted to stall and there was a crunching and grinding noise.
The car could not be pushed in neutral and wouldn't move more than a foot in reverse.
I knew the clutch was ok as it operated and felt as it should, and the gearbox was selecting gears fine and there was no difference to how that felt, so I called out my recovery service requesting a flat bed as the car could not be towed.
Nearly 2 hours later the truck arrived and I explained it all to the driver, and we agreed to stick something under the wheels to enable the car to slide along the tarmac.
As the car was being winched there was a lot of crunching going on and the car was hopping forward with the rear end leaving the ground and spinning off towards the right. We nearly didn't get the rear of the car on the flatbed.
Hours later we make it home and with the help of fairy liquid under the tyres we slide the car off the ramp onto the drive. Oddly, the rear wheels were happy to turn but the fronts remained locked, so that helped us rule out the rear diff which we originally thought was the problem.
Later last night we attempted again to move the car, and it actually drove fine. No sticking, crunching or grinding. We put it down to the oil not being thick enough and decided that we would change all oils today.
This morning after more brainstorming we decided to check all wheel bearings and they seemed fine, however the NSF wheel was much harder to turn than the other 3, and if you turned it backwards it felt like winding up an elastic band, it got tighter and harder to turn.
On inspection the drive shaft is an original part (107k miles) and is crunching away so we are now replacing that tomorrow along with a better 75w fully synthetic gearbox oil as the 75w 90 I put in is incorrect (I wrongly selected it thinking it was correct).
Obviously we incorrectly thought the noise was coming from the rear of the car and mis diagnosed it the first time, but we are confident we now have the problem and will fix it.
So if anyone else hears a whine/howl that they can't pinpoint, have a look at your drive shafts. It may save you a wasted weekend and a lot of sitting around waiting for a recovery truck.
