My truck makes a whistling sound when I travel long distances. The whistle seems to be connected to a speed, but the speed varies. I’ve heard it in the 70s and in the 50s. If I travel faster or slower than the whistle speed, it goes away. I think it might be coming from the engine.
It could just be a wind whistle. If the air moving over the vehicle hits just the right spot at just the right speed and angle it can make a whistling sound, like blowing over the mouth of a coke bottle.
I have such a whistling sound on one of my cars, somewhere in the area of the passenger door mirror. I’ve never been able to locate its source or eliminate it.
I had a similar noise in a 1965 Rambler that I once owned. It turned out to be in the grille. The speed at which the noise would occur would vary somewhat depending on the wind velocity. The 1948-50 Ford pick-up trucks also had a whistling noise in the grille.
But it doesn’t happen every time I drive those speeds.
Do you have cruise control, and if so, does it work correctly? This could be a vacuum leak.