I have a 2012 GS and I'm experiencing a humming sound coming from the rear of the vehicle. No vibrations, just a hum that increases in pitch as I speed up. It's audible from around 35mph and up. I rotated the tires to see if the noise followed a tire and it didn't. I have new brakes and new tires so my first guess would be a wheel bearing. 65K miles, any suggestions?