I've shaken just about everything there is to shake underneath the car, and nothing'* rattling, squeaking, or otherwise. I've revised my initial guess as to when I can hear it -- it'* usually at light load, maybe 1.5k rpms, at almost exactly 40mph every time. (It might happen at all engine speeds, but at 2.5k maybe I pass through 40mph fast enough that I don't hear the rattle?) I've gathered from some other posts I've read that the torque converter clutch locks up at 40mph, right? Maybe it'* related.
But you're right, Steve, it doesn't really sound serious. I'd just like to know what it is, so when it DOES sound serious, I don't have far to look