Originally Posted by geeknerd99

Square taper cranks, I assume? Left crankarm? Could be the square hole on the crank wallowing out. I've seen it happen on a few too many bikes, including my own. I did myself a favor and replaced (upgraded) the crank/BB to a splined system.

Make sure your left crankarm bolt is sufficiently tight. Otherwise, the BB is just gonna make the hole on the crankarm bigger and more out of shape, leading to continuing loosening and eventual replacement by the frustrated rider.