Mine also makes that sound......only when cold and only when reversing. I've kind of learnt to live with it as it happens say only half the time.As the title, if the car is cold, often there is a very loud moaning from the rear brakes as you are trying to reverse. So loud that people look at you wondering whats going on. I'm thinking pads sticking a bit? As soon as you are driving and a bit of heat is generated they are fine.
Its in the dealers today. Last night when dropping it off the receptionist said something along the lines of they all do that!
Of course it hasn't done it for them and an inspection has revealed all looks normal.
Anyone else suffered from this, I have done a search and turned nothing up.
On the plus side, the other noise I described to them from the engine end, they have diagnosed as a faulty DPF and are replacing it under warranty and they are replacing the heater blower fan as it stopped working a couple of weeks ago. Although I had cured it and it was still working, that wasnt a problem