I feel like anything described as a "loud crack" has to be a sudden violent contact of metal on metal. The only thing that makes any sense to me is a chain sort of riding up on top of a tooth, getting some tension on it as you apply the power and then suddenly snapping back down into its proper place when the force of your pedaling overcomes it. Especially with your description of it being preceded by a slight backpedal.
Do you feel anything through the pedals?
Do you feel anything through the pedals?