I have a SRAM Force crankset. Over the last week or so I began to notice an almost knocking feeling on my left downstroke. I checked out the pedals, tightened the crankarms down, checked to make sure the BB was tightened down...

It miffed me until today, when I held the the right crankarm down and tried to move the left arm, and I found a little bit of play up and down. Its not much, maybe half a mm, but certainly enough to notice when pedaling.

Does anyone have any idea what could be causing this? I have the crankarm tightened down. The play is somewhere in that bolt region: