My guess would be a balance issue ... usually there is 2 Colored Dot's on the sidewall of the tire that needs to be aligned to the valve stem ... the dots should be Red and Yellow , the Red dot is the lightest point of the tire and the Yellow dot is the heaviest , how you can tell if it is in balance is loosen the brake pad a bit by prying it away from the rotor slightly to eliminate any drag , make sure the front wheel is off the ground , now gently spin the wheel and watch were it stops in relation to the valve stem (1 full turn is sufficient).. repeat this a few times , if the tire is balanced , it will stop at different locations each time , if it continually stops at the same location or even rocks back and forth to that location then the wheel is out of balance and you need to relocate the dot given where it is in the location , if the red dot is constantly at the bottom and at the valve stem , then move it to the yellow dot , and so forth , but the red or yellow dot should be aligned to the valve stem
Same goes for the rear wheel , but you have to remove the chain as well as loosen the brake to test .... again look at both front/rear and make sure one of the dots is aligned with the valve stem , if you have to , loosen the axle so the wheels really spin freely , this insures a more accurate test result .....
