I have a new 2013 PF 4x4. Today was its first highway drive. I noticed at 70-75mph there was a hum from what seemed like the front left wheel. I didn't notice it at low speed. It seemed to vary in loudness and pitch as i changed lanes. It wasn't the tires or the road. I drove from LA to San Diego and it was there the entire time. It wasn't very loud but was noticeable for a new car buyer. Anyone else have this issue?