I *think* the pumps are mounted on the left side of the engine, not at the rear. My 990 has a fairly pronounced knocking at idle which is a bit more audible at the rear. In my case I believe it's just normal diesel knock which seems louder at the rear of the tractor probably because it is being transmitted through the frame and transmission and the exhaust noise is lower at the rear and so doesn't mask the knock so much there. Also, I sometimes hear a much louder knock which after a bit of searching turned out to be the drawbar rattling in sync with the diesel knock. Just moving the drawbar a bit or putting my foot on it quiets it down. Check the drawbar, 3pt arms, etc. Maybe it's just something rattling like my drawbar.
In any event, a fairly pronounced knock at low rpms is pretty normal for most diesels. As far as the power steering stuttering, I'm not sure ...
Just as a point of interest, apparently a lot of that characteristic diesel knock actually comes from the injection pump itself, not just the firing of the cylinders. My cousin has his heavy duty mechanic ticket (among others) and he said when he was in school they had an injection pump hooked up and being turned with an electric motor. It was just pumping fuel through injectors into a bucket or something and he said it sounded exactly like a real engine running, and just about as loud.
