Well I've just spent about an hour with Apple's tech support and made some real progress. When the help person realized all the things I've tried, he bumped my case up to a higher level techie. To make a long story shot, we've eliminated problems with 10.2.5, my hard drive, my third party RAM. We got the noise to go away by booting into Safe Mode. What this means, is that some application is causing the problem and I've got to start sleuthing there. One suspect is Xsounds, a haxie by Unsanity. I'm going to contact them and see if anyone else is reporting the problem, then take it off my system and see if the clicking goes away. Ron