| The check strap is the bit of metal that looks a bit like a spanner and sticks out of the hinge side of the door and attaches to the door pillar. I'm pretty sure it will be that causing the noise. My sisters car did the same thing. WD40 might work for a short while but it also washes away what little grease there is on there to begin with. You'd be better off getting some spray grease, like the stuff you can get for motorbike chains, and give it a good dose! | 
