It's probably* one of the moveable parts that needs removing, cleaning, re-greasing and reassembling
So it could be the cranks, the bottom bracket fitting in the shell, the pedals fitting in the cranks, the chainring bolts ....the possibilities are endless. Have fun fixing it!
* it might be the stress on the drive train transmitting forces to the saddle and making a noise up there and then the seatpost shoots it back down to the BB area