Hey guys. When I was driving my pre-owned '18 max home from the dealer 2 weeks ago I noticed the same noise. I thought that it was coming from the door/window seal until I put the window down and heard it continuing. Yesterday, I took the front of my hand and placed it in front of the driver side mirror, essentially blocking the flow of air from hitting the mirror directly. It stopped. This is absolutely the source of the noise.
Brought it in to the dealer today for a couple of service issuea, including the sunroof (tech told me "this is an old one, built in '18 and the seals need to be lubed and some screws tightened) and when I mentioned the mirror issue, the tech seemed to know about another TSB that he thought may apply to this truck. The fix involves taking apart the mirror assembly and packing a particular area near the wires with some type of foam, as there's an open channel that creates a wind-tunnel in those mirrors when you hit a certain speed. What's weird is that it doesn't happen below 45MPH or seemingly above 55mph- thinking that perhaps the airflow is too great or too slight for it to channel through the housing and that's why you aren't always hearing it. Curious to see if this is indeed the fix they will apply when I pick it up later. It helps so much when you have a tech who really knows their way around a particular vehicle.