Two thoughts for you to consider. First document the problem and how you
have tried to get the dealership to deal with it. Make sure the
dealership writes up your problem and reports what they have checked, in
writing. Consider leaving the car there overnight. Second,lossen all of
the drive belts and spin the accessories (alternator / AC idler / etc.) by
hand to see if you have a bearing going bad or an accessory binding. I
would also spin all of the wheels by hand to see if I could detect any
noise or roughness. Good luck!