Clicking coming from bottom bracket/crankset?

Any fellow bike techs out there? I can't seem to remedy this problem: '04 K2 Zed team frame, bb shell threads in good shape. There's a single click sound coming from that area every right-hand pedal downstroke under power. I replaced the bottom bracket, (shimano 63x113 english, square taper, lots of fresh grease), tried different clipless pedals, tried the customers' pedals on a different bike, installed a new chain and double checked every link. Crankset offset is good, everything lines up, drivetrain shifts perfectly, torqued crank arm bolts to 39 ft/lbs, but still that click?? Crankset is a Truvativ Stylo, chainrings and teeth are all straight and in good shape. I even checked the cleats on the customers' shoes for wear/tightness and binding. What am I missing? Any ideas you have would be greatly appreciated and thanks in advance!