I'm sorry, should not have scanned your original post so quickly. Totally missed that you had already done the cranks and BB. Low probability but I'm wondering about a frame crack. One way to check for that would be to push sideways on the BB directly from the left side with your foot. That effectively stresses the frame similar to when you pedal.

One other better possibility is the joint between right crank and spider is no longer solid (same with chainring bolt/rivet). If that is the problem one indicator would be that it would NOT occur if you disengaged the chain and then stood on both pedals and bounced up and down (turn 180 degrees and repeat). That removes the spider/chainwheel from the equation, as well as the rear wheel.

p.s. It's not the chain, as only a portion of the chain is under tension at any one point, so the noise would not occur reliably by putting pressure on the cranks.