Tightening boot catch?

Does anyone know how to do this? I've got an awful knocking noise coming from the rear of the car! I've just had it on some axil stands, taken the diffuser off and it's definately not the exhaust!
 
just stick some grease on it (thats what I done), but remember it's there if your putting stuff in the boot. You don't want it all over your clothes :lol: