Chasing vibration

[wilkin250r]

I've got a vibration around 55mph. It's rather high-frequency, much higher than a wheel imbalance, so it's got to be someplace else in the drivetrain.

But it's rhythmic. It's not constant, it comes and goes at a regular rhythm. A fast rhythm, like once a second. It's there, it's gone. Over and over. There, gone, there, gone, there, gone. Bruuup. Bruuup. Bruuup.

A driveshaft vibration wouldn't come and go. It would be constant. Plus, a local driveshaft shop pulled the driveshaft and told me that although the driveshaft is old, and could probably use some attention, they don't believe it's THIS bad.

What else is going to give me a vibration? Especially a vibration that comes and goes.?