I have a 2003 Subaru Forester with about 112,000 miles on it. In late March, while driving on the highway at about 65mph, a rather loud, high-pitched tone/vibration arouse from the rear of the car, towards the passenger side. It sounded somewhat like the flying saucer or death-ray special effects from cheesy 1950s sci-fi movies. It went on for awhile, then finally subsided. It has been recurring intermittently since then. I have taken it to the dealer twice so far–on both occasions I’ve been told–maddeningly, since it happens all the time–= that they couldn’t reproduce the sound. However, they assure me that it’s not my wheel bearings or brakes (which is funny, because as soon as I put pressure on the brake pedal, the noise stops–if I let up, the noise starts again). Does anyone have a clue what’s going on; and how I can get my dealer to hear the noise and fix it? Thanks!
Very wild guess: could this be your fuel pump whining? It has to work harder when you’re driving fast.
A better guess would have to do with your tires.
Thanks–both good suggestions. However, the noise now seems to occur regardless of speed. And the tires are pretty new–only 4-5 months old. It’s got me completely baffled!
the tires are pretty new–only 4-5 months old.
In late March, while driving on the highway at about 65mph, a rather loud, high-pitched tone/vibration arouse from the rear of the car
That would seem to indicate a possible connection. Don't rule out the tyres just because they are newish. Tyres come from the factory with defects. You might try rotating the tyres (front to back and back to front) and see what happens.I’ll give it a try, thanks!
Since you stated that the sound goes away when pressing the brakes this means to me that there is a problem there. I would guess that the pads on the rear passenger side may be rubbing slightly on the rotor. Take the service tech for a ride to show them what you are talking about. This may change things though having another person inside the car.
Thanks, I’m about to call the dealership and insist that the service tech take a ride with me this time, until the noise makes its appearance. Then, I hope, we’ll get this solved! (When I know what it is, I’ll post it here, just in case anyone is curious.)
The clue about the noise going away with brake pressure leads me to change my answer. (I didn’t pay enough attention to this clue earlier.)
Your brake pad is probably vibrating against the caliper or rotor.
I hope to find out on Friday, when I take my service tech for a little ride! 
Thanks for the suggestions!