I had the same problem. I couldn't find the cause, but I think it has something to do with uneven distribution of the weight of the car. When I had the springs on the front sides only, the sound was much louder than after I installed the rear springs. But the noise can still be heard over bumps and uneven roads.
Here's my old thread
noisy-eibach-springs-possible-fix-t568368.html
Some people say the noise should stop in a couple of months after you install the springs.