Potential problem with transfer box in Q7?

[R_A_D]

Hi there

I got Q7 2013 3.0 diesel, 116k miles. Recently I notice a scrubbing noise when I do engine breaking. It can be heard when the speed is below 30mph . Also, I can hear it when I speed up, but only to about 40mph. At higher speed, above 50mph, can't hear at all. Two different garages, told me that it could be transfer box, but none of them is 100% sure. Has anybody experience something similar and could guide me what could be the problem, please. Looks like the noise is located under gear chaining stick. Gears changing is smooth, so I don't think it is a gearbox. 

[Steve Q]

Could it be a sticking brake caliper?