noodles wrote:
........
The only thing I can think it could be is the crank arms moving on the
tapered squares, but this all seems pretty well tightened?

It is possible for cranks to be tight enough to seem and feel OK except
when pedalling, sometimes a clicking noise/sensation results if they were
not fitted tightly enough. I agree with the advice not to repeatedly
tighten but it's worth a second go if you think they might not have been
correctly fitted in the first place. I'm not sure cranks "creak" though.

Leave the bottom bracket until you've checked/eliminated everything else
you can think of. Then be careful any further tightening of the BB
doesn't cause binding which will wear out the bearings quickly (test axle
by hand to feel for any increased friction) -- this can happen even to
cartridge BBs when the shell isn't perfect.

~PB