i had a similar problem on my fixed conversion. I actually ended up replacing the BB. I dont know if this will help you, but for me it made sense because when i bought the cranks from my local bike recycler, they came with a bottom bracket which was "used". When i pulled the old BB it had signs of rust and the bearings didnt feel right when i would spin the spindle. But after the replacement ive had zero creaks and it runs perfectly. The cranks in question where shimano 9-spd ultegras, 6500. hope this helps, what kind of cranks are in question ?