If there is no obvious visible fracture in the fork I believe it to be the carbon giving when you hit the speed hump.
I had a Colnago CLX that used to make a cracking noise when I hit pot holes etc.... It used to totally freak me out but was told it was the carbon doing its thing, Flexing/giving as it is meant to. The impact is transfered from the fork up into the headset making a cacking sound. I do not believe you have anything to worry about.