Yup, one of the wires had come loose from the crimp in the harness. No wonder I soldered all the rest of them. This was the first stepper to get wired into the harness. I replaced the wires with soldered connections and she’s buzzing away.
I haven’t figured out what the rattle is, it is pretty obvious but tough to trace. It seems to be worse on a Y move, but I can’t feel much vibration if I lightly touch the bed while it’s being noisy. I tried rotating each of the rods by 1/4 turn and it didn’t make any noticeable difference. My working theory is either the linear bearings under the bed or the connections between the heater and bed. I suppose it might be something about acceleration or deceleration, but I haven’t tweaked anything there from the original settings.