Since I am a good 500 miles from your car I can't feel the steering wheel.
The steering shaft goes straight to the universal joint, the universal joint is directly connected to the rack & pinion, the rack and pinion is connected to the steering knuckles by tie rod ends the knuckles are connected to the hubs and ball joints, the lower ball joints are connected to the lower control arms.
The power steering motor is connected to the steering column by a gear. If the EPS motor is turned off the motor gives résistance to the steering shaft; thus my guess at the universal joint among all of the other parts that might cause a vibration or rattle.

 

I shook the intermediate shaft yesterday, it seems tight. But, the noise went away when I took the PS fuse out, so...My '88 Silverado had worn u joints on the steering shaft, but this one isn't loose like that.

 

Then it must be something else.