Clicking is most likely the laser unit moving back and forth trying to access a piece of data. If it can't find that data it will move back and forth until it does. In the case of a DRE (Disc Read Error), since the PS2 never finds that data it will continue clicking until you receive the error.
In case clicking is loud, or you hear squeaking, it might be a good idea to clean the slide rods that the laser rides on, and then add some oil or grease.Loud clicking can be the result of warped discs in the drive, check to see if your disc is warped if you get loud clicking.