Well I found out what the rattle on my car was. It wasn't the door as I first thought because it was doing it last night when I took my dog out.

Last night I was under the car for 20 minutes knocking everything to see if I could find it and in the end I gave up. this morning I took it back to the garage and I told them how concerned I was and can they come out with me for a quick drive to hear the noise. Craig (the owner) just walks up to my car and yes "ahh yes, that’s the problem". I looked at him puzzled wondering how he can tell what the noise was without even looking under the car. He taps a metal plate that goes over the nuts for the tires and sure enough there is the noise I've been dreading.

Someone had tried to get the plate off and left it loose. I've never had a car which has had a plate before so never even looked at the tires.

So I feel still really stupid :-)