Make sure the bonnet catch is completely clean and lightly oiled. The adjustment is by altering the peg on the bonnet that engages with the catch mechanism. If you slacken the big nut, then undo the peg with a screwdriver it should catch perfectly, but if you overdo things then the bonnet will rattle.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.