I'm curious, does it howl on full moons or just when your singing in the car?
All kidding aside, my suv does make a howling sound too particularly at speeds > 65 with a crosswind but I don't think it's comes from the mirrors. Over the Thanksgiving weekend, I stumbled across and old thread (by DMOR) about adding screening spine to the front windshield.
If you want to find the thread, search for "screening spline" or "rod backer"
Perhaps screening spline is a MDX-placebo but I have to admit, so far so good. No howling, growling, snarling, not even a wimper...
*Note: if you do decide to add spline, Home Depot has better quality screening spline than Victoria Secrets.