I had the same problem after installing my Koni shocks (which one is leaking…but that is another thread
). It turned out that the bolt holding the top of the shock was loose. Even though it seemed tight on my work bench, the weight of the car compresses the springs when installed resulting in a loose top nut and clunking.
Luckily for me, I was able to get a socket on the nut while in the car and it tightened up nicely. However, if you top nut is just spinning (and not tightening) you are going to need to pull them out to tighten them. And yes, it is very hard to tell how tight the top nut needs to be because you can’t use a standard torque wrench and hold the piston rod at the same time. You just need to proceed with a little trial and error.