My steering rack did the cat trick about 10 months ago. I had to pull the steering rack and replace the belt. Saved $2400 as Ford does not service that component.
My 17 Ford Raptor has the same noise as your truck. It only occurs at slow speeds on deceleration when the truck is cold. It may be the belt inside of the steering rack. It is a small belt inside of the electronic rack and pinion driven by a high power electric motor exactly what you are wondering about. I want you to try the different steering modes and tell me if the noise changes, I notice the noise is less pronounced when the steering is in the Grandma mode or the mode that I believe Ford refers to as "Comfort". My vibration or buzz goes away after 5 min. Of driving. If your truck is mimicking mine, I will pull my rack and put another belt or adjust my belt and let you know. I think it is simple belt adjustment..Thanks again for your help I really appreciate this forum and all of the cool people who are here to help each other out.
I reccomend carrying an extra rack in your spare parts kit when offroad as your truck may be completely disabled by this failure...it will be down by the river flashing Hill Decent Steering Fault Brake Failure