Doug, I just saw this. I was going to tell you it's your adjustable panhard rod.
The "caged" side (chassis/upper/passenger side) is really tough to tighten, and those adjustment nuts have to be T-I-G-H-T. I had to pull the phr down off the car, clench it in a vise, and bang the wrench with a hammer to get that adjustment nut to stop rattling.
Glad you figured it out.