[MichaelXi]

Sure SOUNDS like something loose...???
:dunno:
Maybe re-check the torque?
Especially the center nuts at the tops of the struts and the nuts at the tops of the shocks. DO NOT Over-tighten them, but you might make sure that they are to spec with a torque wrench.
Those particular nuts may not allow for free movement by hand, but may have an impact when the car is settled and moving.
I had something similar once with one of my front strut bolts.
BEWARE, the Center nut takes a decent bit of torque, but if you over-torque one of the 3 bolts that hold the strut in the tower, they are KNOWN to Shear at Very Low Torque.
Do NOT Over-Tighten them!
Keep us posted.

 

[MichaelXi]

If this is the problem the threads may be messed up on the shaft, I could see using a properly sized washer not hurting anything.

I Don't know what you're talking about!
I've NEVER used a washer in that location...
:lol:
The threads on the Koni shafts seem to not go as deep as the OEM dampers, IIRC, so I've Definitely used Washers there to make sure that the nut has Plenty of Good Thread to Bite.