We have the same in ours - the pin has actually ovalised the hole in the body mount, so there is nothing that can be done other than to cut off and replace it, or ream out to accept a larger pin. However, with yours it’d be worth stripping it to see where the wear has occurred. You might be lucky that it is just a damaged pin, or worn check strap itself, as these are easily replaced. If you get down close to the area, you should see what is moving when the door is opened and closed.