Do a POTS adjustment before replacing the laser. My laser used to do the same thing, brand new, and I just turned up the laser strength and it worked prefectly, for over a year. Then, it started clicking again, so guess what, I turned up the laser some more, and it's been perfect ever since.
Here's a rather poor, but best I could find, explanation on how to do it.
http://www.megagames.com/ps2/ps2_las...ibration.shtml
There is more than one way to get to the laser unit, I prefer taking off the top, pressing eject to get the disk tray out of the way, and removing the metal rod guides to get to the laser. Once you get to the back of the laser unit, turn the two screws clockwise, about an eighth of a turn, then give it a test. If it still isn't happening, turn it a little more!