Man, polyurethane is harder than rubber, that will transmit noise. Thicker aftermarket sway bars and LCAs and panhard bars are beefier than OEM and that too will transmit more noise.
In my case the "clunk" is the endlink sliding across the sway bar. I can reproduce it with my hand. Doesn't affect handling, it's annoying but increased NVH just comes with the territory when you're upgrading your suspension.