I have a 2012 dsg and haldex Yeti with 125,000 miles up.  It's recently developed a loud whining noise, apparently from the rear somewhere, but this only occurs on motorways after about 20-30 mins.

If it's accelerating the noise disappears, it fluctuates on a constant throttle, and worsens with throttle off.

Cat is fully serviced and recently had dsg and haldex done. I'm afraid it's possibly a diff or something to do with the haldex. Any ideas?

 

If the haldex has been properly serviced maybe time to have the read diff oil changed.  

Could it be the rubber delaminating in the rear propshaft joint?
 

I hope the person that serviced the Haldex unit drained the oil out of the correct unit! We've seen that mistake before.