Lift the bonnet and thoroughly clean the catch - grease it again afterwards. There is the possibility that the catch is pulling the bonnet down too tight, so slacken the large locknut and unscrew the spike one turn. Tighten up and try again. There is a fine balance between this and the bonnet rattling though. Another thing is to check that the bump rubbers that the bonnet closes onto - sometimes they are adjustable.
John, July 2008