Clicking noise from drivetrain under load
I've dealt with this quite recently.  It happens that the sound was coming from 2 sources, and I was hearing them together. At first it was a constant series of clicks, several per crank rotation.  My first step was to rebuild my pedals.  It was time anyway, so I figured it was a good time.  Rebuild complete, things much quieter, nice!  But not completely quiet-- instead of of the multiple clicks, I now had a single, quiet click.  That is from my crank.  I tightened the non-driveside (the only side that can be tightened on this type) and, voila! quiet.  Unfortunately, it seems that my particular crank has a known problem that requires this from time to time-- every couple hundred miles.  So, until I care enough to replace it, I'll be tightening it a little from time to time.