You probably have a separated tire or your tires are out of balance. If I were you I'd take a trip to Les Schwab and ask them to balance your tires. They will quote you about $25 to balance (and rotate if needed) your tires. But, I bet after they are done up they will wave the fee - doing it for free. See they want you to come back and buy your tires. They did this to me and have gained a customer for life (their service is the best).