Yup - what Miles said...
jack up the car, support it safely, and grab a tire at the 12 & 6 o'clock positions and try to wiggle it - see if there is any movement.
Wheel bearings are ok. No runout at all. It happened again coming home on the expressway, a vibration, and when I applied the brakes a rhythmic grinding timed to wheel rotation. This time it seemed to come from the left front, previously I thought it was right.
Nothing loose with the axles, at all. I've done many axles on subarus and kinda know this stuff.
As I was checking the right front wheel I noticed a little blue plastic cone cap sitting on the control arm edge. It was a cap to a gear oil squeeze bottle. I don't think I ever changed that, but I may have
I checked the front diff and it seemed a little overfilled. 30k change recommended, so I did. The oil that came out was cleanish, I'm letting it settle overnight to see if sparkles settle.
It scares me when the noise happens, I can feel the grind in the brake pedal, but it self-corrects every time.
I replaced the front right wheel bearing, that seemed "maybe" a little notchy. I have seen much worse. I also replaced the left front balljoint which was a surprise that it was loose.
It was fine for a few days, but then the grinding showed up again, and again the brakes ground like crazy. It always occurs at highway speeds, without even touching the brakes, starting gradually, and building up to sounding like a wheel bearing failing. I pull off an exit, make a few turns, maybe it's cooling down, but then the sound is gone.
This is really ticking me off. Could it be an axle? How would the brakes interact with an axle?