I have had this problem before and can't remember if it was on this or another bike.

Quality carbon bike with stiff carbon cranks, well maintained, good strong wheelset.

When I get out of the saddle to climb a fairly steep grade yesterday, I would get this "hooonk....hooonk" with every hard stroke of the left crank/pedal. My wheels are a relatively new, well-maintained and strong custom build with Chris King hubs, so I wouldn't expect a hub issue. I also maintained the cranks not too long ago, re-greasing the threads and spindle and re-installing cranks, so I wouldn't expect a failed BB bearing or the like.

It almost sounds like the sound of braking but I would be surprised if my stiff wheelset were flexing enough to rub against the pads, but maybe....

Ideas?