Thanks for the advice! I checked that last bolt on the PS Pump bracket (bracket to PS pump)... the one that's hidden away and can only be seen from the bottom. That's tight. I'll try looking for the ones you all suggested tomorrow.
I did have an opportunity to do some more checking (once I determined that it wasn't an engine issue, I drove to work. One day at work pays for a new (to me) alternator or PS pump... so I figured it was a win). The rattle can also be caused by turning the steering wheel sharply. Now, does that point toward the PS pump? Or is it the alternator being affected by the PS pump engaging, since they're connected by a belt?
A couple more clues to the puzzle:
-The PS pump belt that goes to the alternator is much tighter than the other PS pump belt (the one that doesn't go to the alternator). Does that have something to do with it? I'll have to keep messing with them to try to get the tensions more equal.
-My truck consumed 4 quarts of oil since October. (my commute is 60 miles per day, 5 days a week, so the truck gets a lot of miles. Still seems like a lot). It doesn't seem to be burning it... there are a few apparent leaks (Class 2), so this might not have much to do with the rattling issue, but worth mentioning. I replaced the CDR valve recently, so that's not the problem.
- The PS pump fluid was pretty low. I topped it off, but haven't driven the truck since. Will report back on that tomorrow.