Bass String Buzz @ 1st & 2nd Fret
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I had the same issue with one of mine many moons ago. It just about drove me nuts because for awhile everything was fine and then the buzz thing would crank up. I finally figured it out when like Ken points out, it was me catching the string on the strum and pulling it up a bit. I also put a bit of a heavier string on the bass then I found was included in the string sets you buy. As I recall I went from a 24 or so to a 28. (I think the Strother string calculator is a bit light.) That helped a great deal but it still happened. Steve Siefert once mentioned that a heavier string could correct some problems like this when using a 155 tuning.
The next time it happened I froze on the chord and it dawned on me that my bass string was not being held with the same amount of pressure as the others allowing it to buzz......
I am not saying that your issue is the same, but in my case it was due to several different things.......too light a string, a weak ring finger and an inconsistent pick angle. I also switched to a lighter gauge pick too. That made it harder to lift the bass string.
As suggestion, if you have not tried this, tune it up a to maybe EBE and see if the higher tension makes it stop.
All in all, Ken's comment fit my predicament and was right on the money.