GMC Acadia RPM needle moves and not stable in idel situation

I recently purchased a new GMC Acadia in Australia, From the very first week, I
have been observing that the car has a little vibration felt in the steering and
RPM needle moves very slowly moves up & down when in idle and
Neutral/parked. I have taken it to the dealership couple of times and they insist
that this is  standard and no fault. I am concerned if the very new car is having
this issue, what I am going to face in future. Other thing that I have observed,
you get the feeling while driving  as if the car window is open whereas it is not.
Appreciate any insight and what should be my next step