At the end you say, “then it went away.” Does that mean the sound stopped occurring as you increased to even higher speeds? Or that it completely stopped and didn’t return the next time you were at that 34+ MPH range or ever since?
When I’ve had imbalanced wheels, I could feel the vibration through the steering wheel. When my CV joints went bad, the sound was worse when going around a curve and the wheel was bearing more weight – I also felt the vibration, and it was more pronounced at lower speeds. Admittedly, neither of those problems has occurred in a couple decades, so maybe design changes have resulted in different symptoms, but it seems such a vibration would occur consistently and worsen until it is fixed – it wouldn’t intermittently go away.
If you did not noticing any changes in how the vehicle performs, it could be as simple as some uneven wear on the tire, if you’ve had to brake hard a few times or hit a curb, for example, but that would also be consistent. If the sound is mow gone completely, you might have picked up some road debris that is rubbing against something, or a stone in the tire tread. Or, as others have noted, it could be caused by something way more serious.
Looks like you’ll have to get a good look underneath to get a better idea. Good luck, and please report back on what is found to be the culprit!