Ya thats the focus, I have taken about 20,000 pics with the a75 so far and I am now sure that the clicking is the lense actually foccusing. Zoom will have that little motor whine, bur if you watch the LCD, notice that it only clicks when you are foccusing.
If you really want to try, put camera in MF, zoom all the way out and run the MF from one end of its range through to the other without stopping, you will notice that it clicks when it tries to focus. The camera focuses alot faster when it on AF so the clicking sounds worse, but its actually just running through a wide range much quicker then it would on MF.
Cheers, Its such a common thing that I guess it normal for these cameras to do this, since my camera is running beautifully.