Steer axle bearing.

What model of truck? Look in owners manual or visit a dealer who sells them new. If it has a large plastic nut, usually the bearings will be greased at factory and have a recommended time or mileage to inspect and grease and put back together. As a start, jack the axle off the ground and push in on the top of the wheel to check for a loose bearing. Also check for loose parts like tie rod ends, drag link, steering sector. Sometimes you can use a long bar to get more leverage to check for loose bearings.

 

those are supposed maintenance free hubs. no need to grease or do anything with them just replace whole assembly when it goes bad, i suggest if one goes just replace both and you have a piece of mind

 

What axle and hub do you have? If it is a unitized hub, you check play and make sure it spins smoothly. If the seal is leaking, any excess play and/or bearing noise means it's time to replace the hub assembly. If it's a conventional hub that is grease packed (which I rarely see on steers) then it's the same but you can replace bearings.

 

I think he might have a smaller truck, or an older truck, like one of those Isuzu's.

 

Need to spin without the wheel on and feel it with your hand. If it doesn't feel smooth it's needing changed.

Otherwise it is most likely another problem, maybe shocks bad and/or wheel that is bent...or tire out of balance.

What brand, model & size steer tire is on it?