Steering wheel rocking?

Hi, I have a 2010 Nissan Altima with 124k miles. For the most part it has been reliable minus fuel pump needing changed a year ago. My wife drives the car and didn't tell me but I noticed today while driving it that when driving around 20-40mph the steering wheel rocks back and forth and then fades away with... show more Hi, I have a 2010 Nissan Altima with 124k miles. For the most part it has been reliable minus fuel pump needing changed a year ago. My wife drives the car and didn't tell me but I noticed today while driving it that when driving around 20-40mph the steering wheel rocks back and forth and then fades away with higher speeds. I plan on taking it in. What does it sound like it could be??? One tire was replaced, maybe they need rotating? I hear things like it could be transmission but there have been no signs about it going out.
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