Possibly warped rotors, loose wheel bearing, or a defect in a tire. Inspect your tires carefully, looking for bulges, or any other abnormality. If you find none, then you will need to check the wheel...
Could be a minor out of balance condition with one of the tires, or a minor alignment issue that is unnoticeable at lower speeds. Also the tires may not be rated for the speeds you are attempting and...