Check and lube your saddle rails - not the seat post, but the rails where the saddle attaches to the post.

I had the exact same thing on my bike: click when left foot down in pedalstroke. Sounded like the whole transmission was falling off. Through trial and error in the wee dark hours of the morning of a double century I diagnosed it as being the saddle rails. (Easy diagnosis: Pedal while standing out of the saddle on a flat road with resistance on the chain. That'll tell you if this is it.)

LOL about the knee click!