When I play open A acoustically I get a loud rattle. It's not fret rattle because if I put my finger on the string between the nut and tuning post it disappears. My first thought was that the break angle over the nut was too shallow, but the string is as far down on the tuning post as it will go. I've tried a different string but it still has the same problem. Could the string be too loose in the nut? I understood that the string must be free to move through the nut and that the string tension should be even from the bridge to the tuning post, with the break angle over the nut just providing the right speaking length. Can anyone explain this to me?