i second bostonfixed's suggestion about the bottom bracket. the fact that the creak responds to pressure suggests shear stress between the bottom bracket shell and the cups. i had a creak like that was infuriating me, not chainring bolts, not crank bolts, not pedals. i finally disassembled the bottom bracket and it had gotten a little dry and crusty between the cups, which are alloy, and the shell, which is steel. a little wire brushing and light greasing and i can sneak up on anybody now.