I had a knock that I was convinced was from my UN-55 bottom bracket that was only 1000km old at the time. I fitted a new one, and still and the knock reappeared after a couple of days again!
Then I wondered if it was from the pedals and not the BB. And that was exactly what it was! After cleaning and regreasing the pedals, the knock disappeared for a few months. And when it reappeared, I already had new pedals ready to replace them with.
When my teenage son presented his bike with a similar problem, I was convinced his must be the pedals too. But in his case, pedals didn't help. It was really the BB bearings on his that were shot and needed replacing.
Good luck in figuring it out. But don't discount bolts needing tightening, or other moving parts as potential source of the problems. And when you figure it out- post your solution into this knowledgebase.