Something clicks while pedaling
A few things to do to isolate the problem to the bottom bracket:


Make sure you also hear the noise when standing up (Yes, I know you replaced the seatpost, but it might be the seat. Maybe.)
When you pedal hard, you also exert more force on the handlebar. Try yanking the handlebar every which way while stopped. See if you hear anything.
A while back I had a frame with a crack in it, and I'd hear a creaking sound when pedaling hard, always from the same pedal position. But, a clicking sound like you're describing probably wouldn't come from a frame fracture.


Bottom brackets (especially cartridge units) can make strange sounds when they're starting to wear out (even if they feel fine). That's what my money's on.