In all seriousness , please forgive my little bit of humour earlier in this thread : I was just trying to lighten the mood , not to poke fun at you .
I'm guessing your car is still relatively new ( at least compared to any of mine it will be ) , do you still take it to an official M-B dealer for servicing ? If so , and in line with Black C55's suggestion , might I suggest you take the car into your dealership , with your son present in the passenger seat , and get one of their technicians to come out in the car , with YOU driving , and demonstrate the issue to them , and ask him to diagnose the cause .
If your car is a little bit older , and you now get a reputable independent place to look after it for you , the same advice still applies - take the car in , with your son present , and demonstrate the issue .
As a preliminary to either issue , while you are driving , can you get your son to capture the noise on a video recording ? Maybe you can share the video with whichever place you would be taking the car into , so they can perhaps have some ideas ?
If the place where your car is still looked after is also the place you bought it from , I'd suggest that might give you a little more leverage ...