Just wanted to tag along on this thread since mine is doing this as well. The loud 'siren' only happens for a minute or so on a cold morning, but the faint whistle goes on and off while driving still. It definitely sounds like some sort of air turbulence whether from a vacuum line or the airbox. But it's just too cold for me to dig around to find it. And has been happening for a few months now so I don't think it is really affecting the driving of the truck.