shoulda bet ya...

 

Possibly a tire with minor tread separation. I've had this happen many times to me. It'll start with vibration at high speeds and soon you'll notice a lump in one of your tires and the vibe will be more apparent at lower speeds. If you put it off, eventually your tire will blow if tread separation is indeed your problem. I definately agree that something is probably wrong with a wheel/tire though.