I took a peek at the serp belt after I got home yesterday. The noise was active, but not as loud as it can get. The tensioner was not bouncing; in fact, it was almost stationary. So I don't think it's the alternator pulley.
However, the whole motor was vibrating back & forth pretty good. It was easiest to see by looking at the loop for the oil dipstick handle, fuel lines, and the emblem on the cover. Like, the dipstick handle looked like a blur, if that makes sense. Moving back & forth about 3/8". You'd never be able to see if there were any bubbles in the fuel supply line with it shaking like it was. But I didn't feel this in the car. Broke out the stethescope, and the PS pump and the tensioner were quiet but I didn't have time to go any deeper.
I popped the hood when I started it this morning, and the motor was running normally, with no noise and no vibration like above. But as I was idling at a stoplight after driving a mile or so, the noise was present.
So I have noticed that the rattle seems worst at idle when the engine is warming up. I may have to start a thread on this, but I want to do a little more diagnosing tomorrow beforehand.