Hey Dan,
I know with mine they supply shims so that the door rests against the back door on the rubber stop if the carrier rattles the rubber isnt being forced against the door so it is not fitted correctly. make sure your door seal is fitted correclty before shimming the carrier.
Mine locks in the closed position and open position, basically a sprung block falls into the base hinge plate at the open and closed cutouts.
There are different types of carrier so would help if you explain how it locks.
Jai