Welcome to the world of coilovers; a more complex strut with components designed to be adjustable which in return can cause noise if/when things get loose. Noise can happen from time to time whether it is a $500.00 or $5,000.00 coilover. Sounds are often caused by some part of the coilover becoming loose over time.
Things to Check:
1) Top 19mm Nut: This top 19mm nut secures the shaft to the top plate/ pillowball configuration. Over time it may become loose and cause a clunk when going over bumps due to play. How do you fix this? 1) Take a 19mm wrench and place it onto the 19mm nut. 2) Hold a 5mm allen key tool in the actual shaft of the coilover (make sure to unscrew the little dampening clicker on the top of the strut and place it to the side). 3) While holding the shaft still with the 5mm allen key turn the 19mm wrench clockwise; this will tighten the 19mm nut and mate the shaft/pillowball assembly tight. This can be done in about 30 seconds per strut, while it is installed on the vehicle. 4) Put the damping clicker back on. For a more detailed video see here.
DO NOT TIGHTEN 19mm TOP NUT USING AN IMPACT GUN – the force of an impact gun can not only damage the coilover internally, but can break the top of the shaft of your coilover!!!
2) Check your adjustments: If your preload is incorrect you will get noise, make sure it is to spec (check preload adjustment article above).
3) Check all other Parts: double check your collars, mounts and anything that can become loose.