2008 Suzuki Grand Vitari V6 4WD. What could be causing a harmonic noise upon reaching 50 to 60 mph? It is intermittent and can last for hours on a long trip. It doesn’t matter if AC is on. It sounds like a piece of plastic is stuck in the air vents, a high pitch.
Check the trim around your windshield. Be very thorough because one little piece can make a whole lot of noise at highway speeds.