+ I would check for seized ball joints even though they should be making a noise, just to be sure. The steering rack can be leaking internally, the actuator part is basically a hydraulic piston, even without showing external fluid loss. IF you have the variable assist rack (should be extra wire) could be something going on there. And yes, check basics like slipping belt first !

If there is something else more diesel specific maybe one of the diesel guys could chime in