^^^^ +2 on the wheel bearings
Also seen vibration a one speed on a tire and wheel that balance perfectly but are not concentric with the axle. This causes loping. The tire and wheel can be perfectly balanced, perfectly circular, but, not concentric with the axle. In other words the center of the diameter of the wheel and the axle are not the same point. This is called eccentricity (like a lobe on a cam). Check this by using a fixed point, like the fender tip, and hold a pencil against the fender tip so it just hits the center of the tire. Now spin the tire and see if it pushes the pencil away or a gap appears. If it does and it is a cast wheel the tire is bad from the manufacturer. If it is a laced wheel (spokes) the rim may need to be tuned by a professional (he/she will have to loosen the spokes to recenter the hub).
