At speed? Then I'd say first thing is to have a tire shop check the lug nuts to see if they are loose, then the balance and condition of the tires. Then have them check the alignment . If all checks out, then have them inspect the suspension for worn components. Worn brake rotors can also be suspect, especially if you experience more vibration when you step on the brakes. Good Luck!
If it makes a knocking sound check the steering coupling in the power steering motor just replaced mine it was shredded
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