I had the same issue... couldn't figure it out... it only clicked on the down-stroke of my right foot, under pressure. Adjustments to the front deraileur made not difference, gear choice made no difference, crank & pedals felt tight - no play. It would come & go a bit.
This morning I decided to give the pedals another look. They still felt snug, but I was able to tighten the right pedal a very slight bit. This fixed the problem.
The sound it was making was more like the crank hitting the end of a cable or something... not what I would have thought a slightly loose pedal would sound like.