Flex, no, it only takes a small bit of movement to create that "Clunk". The bolts don't hold the arms or track bar in place, the pressure that the bolts exert on the mount squeeze the bushing between the mounting plates and make the joint solid. If you failed to torque the bolts enough, the bushing will move, and you will hear a "Clunk".
Because it is a unibody, the "noise" will travel though out the chassis. You just happen to be sitting near the mounting point of the front hardware.
Check both upper and lower Trackbar mounting points. Make sure upper point is not wallowed out where the stud goes though. If you replaced the upper mount, make sure it is torqued correctly.
Check both upper and lower CA mounting points, at the axle and the chassis.
The adjustment points Larsen points out are on the Chassis side of the LCA. I have not seen them drop out of the mount on an install, but if they came up missing (like you actually removed them on purpose), the LCA mount would not be a hole, but a slot.