I'm used to single rider bikes, but new to tandems. We have a specific creak that I (as captain) can also feel through the pedals. It's on every strong stroke of the captain's pedals, left or right, but if I take my feet off and let my stoker pedal, it doesn't happen. She also can't feel it reliably through the pedals when I cause it. But she is only about 1/3 my weight (also 1/5 my age, and inexperienced).
The bike has an eccentric front BB, and the creaking started a few tens of km after I had to change the front crankset (the previous owner is not to be trusted with screw threads). I've already made sure the cranks and pedals are on tight. I plan to check the chainring bolts, but could the BB cause this? Is there something else tandem-specific I should look at while I'm at it?