I have a 2005 Lexus GX470 that is making a lot of road/wind noise coming from the rear of the vehicle. The tires were recently replaced, but the noise was noticable before this and new tires did not help. The noise is more apparent the higher the speed I am traveling. I took the car to two mechanics, one being a Lexus dealership and was told this was not out of the oridinary road noise, but I know it's louder than usual. The dealership did not look under the car. What could be causing this? My gas mileage also seems to be down 2-3 miles per gallon since this problem started.