That is exactly where my sound is coming from also. It is super loud while the truck is sitting in my garage. I can even hear it with the windows rolled up. It is much harder to hear when the truck is out in an open parking lot. You pretty much have to crawl under the truck to hear it. I think that is why the mechanic didn't pay much attention to it. To me it sounds like a stuck relay or bad capacitor or some other electrical component. Mine has been doing this for over a month with no apparent problems. It bothers me, but I don't know what else to do about it unless it actually causes a failure for something. I'm eager to see if your dealer acknowledges it or not.
Again, the sound starts the moment you press the ignition button (without pressing the brakes), and doesn't stop until you turn the ignition off. I first noticed it around 2K miles. I now have about 3.5K miles. No modifications have been made to the truck.
