Mabe they didn't ballance the wheels right..... whenever you have a rattle or shake or vibration ANYWHERE on a car, you should have all 4 wheels ballance checked. They can also tell you alot at that point as well: if you have a slipped belt in the tire, if the rim is bent, or if there are any bulges. But before you do anything, have the wheel ballance checked on all four. Even if the tires were just replaced..... they may not have done it right, or a weight may have fallen off already. Those are always possibilities.