could be a stiff link or two on the chain
Highly likely to be stiff chain links, caused by not maintaining/lubing enough, that just rust over time.
Not sure what i'm s'posed to be looking at in the photo, but if you look at the lower part of the chain from the side, stiff links will not be totally horizontal, they'll be at a slight angle.
Even one stiff link can cause the knocking on the left foot.
Chances are you're gonna need a new chain & sprocket set.