I've got 6,600 miles on the Focus. Two days ago, I rotated the tires myself [in the manner recommended by the manual]. I confirmed all of the lugnuts were secure when I lowered the car off the jack, yet ever since then I've noticed a _slight_ cyclic low-pitched vibration from the front. The steering wheel does not shake at all, even under braking, nor is handling affected in any noticeable manner. What could this be?