But the noise seems to have come from the end link nut coming slightly loose. I tightened it and the noise went away. However, after a while it came back. I assume it loosened again (despite me torquing to spec). The bars were set to full stiffest. I will set them to soft and see if the nut loosens again.
I find that endlinks need to be retorqued after a couple hundred miles any time you have them apart. If they loosen again after that, maybe add a lock washer.
For the record, it seems that the only time that I was able to get the back end to swing around is when I turned off the traction control and *really* turned left (or right) really hard. I felt the outer rear tire come off the ground and come back down. I have all stock suspension except for the Neuspeed 28mm hollow rear sway bar.
Turns out the internet likes to exaggerate the effects of some things.