2004 Subaru Forester

Above 70mph, a squealing noise begins to come through the dash air vents intermittently. At 80mph the noise is constant. This occurs whether the outside air is circulating or inside air mode is selected.

Just a WAG here. It may be how the air is running over the plastic piece at the bottom of the windshield. It is might be near the air intake and that is why you hear it from the dash vents.