mmmh most people take a hair dryer to it for a good while and that ease's up the locktick, then you wana get a rench or sumit, just make sure you dont bash any other bits of the gun
Get a towl or somthing and rap it round then get a " mole grip" as my dad says basicly just a big pair of pliers and grip it tight and turn it.
As for the rattleing does the gun work fine? including the eyes? it could be the eye wire has come out and is rattleing , only thing i can think f that would rattle oooooooooor it could be the trigger hold it when u shake and see if it is.