cyclical purring vibration 2003 Impreza at 70mph

[Tracy]

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

 

[MK IV]

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

I'd say wheel balancing. I had a similar problem. Have someone competent
balance your wheels.