2014 VW Tiguan: chirping sound from the back axle

What is your question?

I have been hearing a chirping sound coming from the back axle, it happens at various speeds, but mostly on highways when I do above 100 Km/h. I thought it was the brakes, so I took the car to the local VW dealer, they told me the brakes were fine, that I had still 50% of my brake capacity left. They said that sound could be the rotor. Usually the cabin is very quiet, but this chirping noise makes the radio hard to listen to. I'm looking for a second opinion on this matter. Thx