Whining in rear

I have an '07 Dodge Nitro R/T 4.0 with auto trans. When we test drove the car it had a slight whine in the rear. The salesman brushed it off as exhaust. We bought the vehicle and it has got worse especially at 60mph and faster with cruise on or holding steady at cruising speed. The vehicle now has 3000 miles. Already had it back to the dealer and the mechanic only drove it on the lot and said sounded normal. Only the service manager could take it out on the road. I have made another appointment for the service manager to check it out, but I need some answers as what it could be before dealing with the service manager.