Try this... Pluck your open A string, and touch the A string BEHIND the nut. Some A strings on many basses rattle behind the nut because they are not wound down around the tuning post far enuff, so the A string will rattle or buzz in its nut slot, even when fretting notes. I'm not saying the gentlemen above aren't correct. But, I have had A strings have an unexplained buzz that turned out to have nothing to do with frets or neck relief. Pushing down gently behind the nut while the string is vibrating raises the string just past the nut slightly, as well. A test to determine if nut slots are filed too deep. Let the string ring, push gently behind the nut, and if the buzz goes away, it's a clue that a nut slot is too deep. I fix this temporarily by putting a folded piece of paper under the string in the nut slot. I can raise the string in very small increments this way, get rid of the buzz and enjoy the instrument until a new nut is cut or the slot is properly filled. I hope it's just a relief adjustment, but it could be other things. Hope this helps.
