It's the Upper Control Arms (UCA) making contact with the coil bucket. The UCA is basically what limits your downward travel on the X, and makes the noise as well. The common (But expensive) fix is to replace the stock UCA with the Total Chaos UCAs. This will give about 1" more of travel, and provides a rubber bump stop so when contact is made, it's not anything you will even hear from inside the car like you do now.