Hmmm. I used a torque wrench when I installed the adapters, but then took them back off and saw marks on the wheels; apparantly when you are tired the stud looks short enough before it actually IS short enough. I ground them down enough (and carefully tested each one) prior to reinstall, but have have this minor vibration ever since I installed the spacers.
I also forgot to track the orientation of each tire on the hub (seriously, I should have done this on a different day!), so they probably didn't go back on in the same orientation as they came off. Some years ago you'd do the tire/wheel/hub with it still on the car, and it mattered which way the wheel went back on. They don't do that anymore?