Your description of the bb is a bit ambiguous. It's a square taper but is it a sealed cartridge or a cup-and-cone type?
One common source of creaking is where the cups thread into the bottom bracket shell. Grease or teflon tape along with proper torque usually solves this.
Another possible problem is worn bearings if a sealed cartridge or bearing adjustment if a cup-and-cone. Cartridges must be replaced but cup-and-cone can be serviced.
Finally, be sure the crank arms have been installed and torqued properly on the spindle. Inadequate torque can let the arms loosen and give the same symptoms.