That saddle/Seat post creak can appear anywhere on the bike when you are riding it. Often mistaken for crank noise because as you pedal- weight is transferred from the pedals to your body and it can be the weight shift on the saddle that makes you think the noise is crank related.

First things I check are for loose BB- Pedal noise from the bearings and then saddle. After that it is down to observation and elimination.