A brand new car that sound like a very old car

I bought a new Mercedes Benz Vito Shuttle last year May / June at Sandown Motors Centurion. The car had a funny loud noise from day 1 when I was mainly turning and reversing. I reported the matter to the dealership immediately. They promised me that the problem is minor and they will fix it. The car has been to the dealership more than 20 times for the same problem. Every time I took the car to them, they either claimed that they couldn't hear the noise or have fixed the problem. I have never experience such a bad service in my life. Early this year, the dealership promised to involve another dealership to fix the problem. They promised that should they not be able to fix the problem they are going to settle my car. The dealerships could not fix the problem as the noise is still there. What is shocking is the fact that Sandown Motors now want me to settle the deficit of about R100 000. They are making a huge U-turn saying they will only pay for the full trade-in value. This is cheating at its highest level. I did not decide to sell the car. Brand new car with so many problem. This is a disgrace to the brand Mercedes Benz. My family & I have suffered because of this.