Maybe you are lucky and the coring is rotted out. Maybe you're not and the stringers are coming loose. You need to access the underside of the step down area and find out for sure what the damage is. Be very sure that the stringers are not involved.
Tieing the stepdown to the stringers, if its not already, would be a good idea, if done correctly. I'd be very careful about tieing anything directly into the inside surface of the hull. Need to be careful of creating hard spots or otherwise causing flexing where its not wanted.
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