Xc40
 

I'm a new owner and just bought an XC40; love the car EXCEPT there is a strange slight noise between 0-50mph (road noise overtakes after about 50mph; it's not the engine noise but becomes more pronounced if you barely touch the accelerator.
I went back to dealer and the same noise comes up on a test car. I then drove XC60 and there is no noise at all.... any ideas what it is?