At my 13th fret on my acoustic, when i fret it and play it, there is a very very light sound. If I move to another fret on the same string, the note rings fine. I'm not sure what the issue is, could it be action or uneven frets?
From the brief description it could be either. You can rule out uneven frets by using a straight edge and laying it across the frets at different places and see if it rocks at all. If it rocks that means it is sitting high on some fret.