You know I had the exact same issue with a Centaur BB/crank combo. My LBS where I purchased the bike took it apart twice over 4 years to try to get at the problem, never could fix it. The bike was then stolen and the frame recovered, and I rebuilt it with a Record crank and Record carbon BB, and I just realized on a ride a few days ago, that annoying sound is gone, I hope forever.
The guys at my shop speculated over some cold Sierra Nevadas and pizza that the problem had to do with the fit of the crank arms over the square taper not being perfect, and that no amount of tightening will ever completely eliminate the play.
Solution - new crank and BB.