I have noticed a clunk mid pedal stroke on my bike. There is a bit of play in the bb (I think... I have described the issue in the following sentences). While clipped in to my pedals, I can move the cranks side to side a few millimeters in each direction. Each lateral move of the crank is accompanied by a clunk. I can feel it in my legs and I can hear it. Are the cranks loose on the bb spindle, or is the bb loose?

Edit: I have a multi (3?) piece crank with 9/16" pedal threads... If that helps...