[Grant@Tirerack]

That could be any number of things. If it is on the same corner where the patched tire is, I'd have the tire pulled off the wheel and inspected. You may have a failure in the patched area of the tire. If it were a bent wheel the vibration would be fairly constant and would not come and go. If the interior of the tire starts to break down, it may be causing the rumble as the tire gets to that speed and the layers of the tire are under a specific amount of stress. I'd get that pulled and inspected (off the rim) for damage!