I have spent all day trying to remedy this problem, including adjusting bridge height and swapping strings. The A string on my bass buzzes incredibly loud, but only when playing C, C#, D, using the fourth finger (i.e.) C- half pos, C#- 1st pos, D- 2nd pos. The buzzing completely vanishes when the notes are played with either the first or second finger. I have been playing this bass for nearly 3 years now and have never had this problem. I DID recently lower my string height VERY slightly via needle file, but as I said, I have tried raising and lowering the action via bridge adjusters. I have tried altering my fingering technique, i.e. spacing of individual fingers and application of pressure to individual fingers whilst in the mentioned positions, but the buzzing persists. Money and time prevent me from taking it to a luthier in the near future, and I am slowly going insane while practicing. Please help! Thanks.