I have been going crazy recently because of a click I occasionally heard that sounded like it was coming from the left side of the bottom bracket. It only happened when I was pedaling, and always when my left leg was on the downstroke. I couldn't find it with the bike on the stand, and the sporadic nature of the sound was driving me nuts (or more nuts). I even asked a co-worker to ride the bike to help me pin point it. He didn't hear it at all!
Well to make a long story short, it turned out to be a click in my knee!!!!!
Arrgghh, getting old is the pits!
Hmm, I wonder if White Lightning would work on my knee?