Yes, when I reproduce it I am basically off the bike standing on left pedal. Either smashing the pedal from a complete stop and rolling forward, or someone holding the bike and bouncing on the pedal. On the road its when I mash hard, or resist hard (its a fixed gear) is when it happens. Annoying, and can feel it through the pedal into my foot. Always left side to make the sound.

Also, when I tightened it and heard that sound, it was almost identical to the creaking sound. So its something definitely in that area.

When I put the crank arm parallel with the seat tube and squeeze, I hear no sound, there is movement, but just seems like the crank flexing. Definitely not rocking / loose /creaking when moving by hand.