Which dealership looked at your car. My dealership claims they cannot hear it.Had this same "fog horn" sound on mine, started hearing it right about 50 mph...Service manager test drove the car and heard it as well, but couldn't figure it out. After taking a close look at the front end, the service manager and myself noticed that the hood and fenders did not line up just right, leaving a gap and over hang on one side, and the bump stops where not set at the right height. He had the body shop take a look at it. They where able to reset the fenders so they both a nice tight seam to the hood, and they also found that the hood latch needed to be lowered, and then reset the bump stops to the correct height. Once all that was done, there was no more "fog horn" sound at and above 50 mph...yeah it should be done right at the factory on a car that cost me 70k, but it is done now and all is well with it...