Strange flappa sound

okay first off, some info

it's a 4 string bass with these strings and I tune them to C standard, all 1 step up from BEAD.

The "C" string (.130) has a strange flapping rattle sound but only when playing open or at the first fret. Tuning further up or down makes no difference. I filed the nut for this size string using a drill bit and some sandpaper, not the most accurate way but it sits in there pretty snug and it's not too low. First thought was bridge, upon holding things down and trying to eliminate things that could cause it I don't think that's it. I filed the groove a bit anyway and the string is snug in the bridge but not too snug.

Next theory was the truss rod vibrating inside the neck (from the way it sounds, it really sounds like this is the problem). I checked the truss rod and it wasn't under any tension, so I tightened it up until it just started to grab, problem was still there. So then I cranked it in a couple of turns just to see what happens, problem is still there. So I got it back to the amount of relief I like it and have left it alone.

Is this a bad string where the winding has seperated from the core? I don't have any spare .130s to test it with, but when the bass was strung with std guage (.105) and in E standard there was no issue. Doesn't occur on any other string.


It sounds the same as this

It sounds like it is coming from the nut or the headstock in general but its very hard to tell because you can still hear it all over the guitar.

Thanks.

edit:

forgot to add, can't hear it at all through amp. Just acoustically.

Also tapping the neck or shaking it, theres no sound, the only way to get the sound to occur is to pluck the string.

 

Just put the string on another bass, same issue.

Must be a bad string.

 

thanks very much jason!