Could be a number of things. I'm inclined to scope the pedals first, especially given your repro steps...I've had inexpensive pedals (Forte) have spindle creek/ticks when standing up on them...My Looks, Ritcheys, and Speedplay's never had the noise...Second spot would be the bottom bracket or cranks...from there, it becomes a lot trickier. Since you're saying it happens under load, I don't think it can be too far from the pedal train in general.