Slight periodic vibration at highway speeds

[Christien]

I just noticed this yesterday and the day before. A slight vibration that comes and goes very regularly at highway speeds. I can hear it and feel it in the steering wheel - on for about 1 second, off for about 1 second. Very regular. If I shift into neutral it doesn't go away, so I don't think it has anything to do with the engine. My thought was maybe tires? The left rear is slightly newer than the right rear - I had a blowout last year and replaced just the one tire, though exactly the same tire. Could that be causing the problem, and if so, is it anything to worry about, or just an annoyance?