cyclical purring vibration 2003 Impreza at 70mph

Driving home my (pre-owned) 2003 Impreza Outback Sport, I noticed a cyclical vibration when the car reached about 70mph on the open highway. It was barely noticeable....like a cat purring. Definitely cyclical: a very slight shuddering for a few seconds, a couple seconds of silence, and then the vibration again. If I slowed down to about 65mph, it stopped. The road surface changed from blacktop to white concrete but this did not affect the vibration, so I assume the vibration was not casued by the road surface. What could be causing it? I had been running the A/C but turned it off when I noticed the vibration. Thanks Tracy