Unbalanced tires and/or you are needing a wheel alignment.
around 50 bucks for a wheel alignment (I would start here.. is it a new or newer car you have?)
Most tires unbalance over time, but usually isnt that noticable unless they are really worn or the wheel weights have fallen or been knocked off.
I would really have to know more and possibly see what you mean to make a firm assumption.. but Iwould point my finger to wheel alignment adn unbalanced tires.
PBM is getting tired... and drink is finished.