Originally Posted by XMilkCrateX

I had the same problem when I got my bike, and I would of sworn it was my crank, but it wasn't. Just take it to a bike shop and have them tighten the cog and lockring. Then sprint real quick near the shop and don't apply any back pressure at all it. Hopefully you have a front brake or a steep hill near by. Then tighten it again. Hopefully you have tools to tighten the cranks if not get some.