Originally Posted by djeucalyptus

is it a click or a quick metallic creak? most of the suggestions above are solid places to check.

My first guess, from what you suggested, is that either your cranks are slightly loose or either a crank or your bb has become slightly deformed. Square taper, yes? If it's slightly loose (or slightly trashed out of square), a hard pedal stroke will torque the crank in one direction, while a hard backpedal will torque the crank in the opposite direction, unsettling the slightly lose joint, and causing a metallic clicking sound where there's a slight shift between the two planes of contact. Wrench down on the cranks and see if it fixes the problem.