Originally Posted by The Rage
I realized the rattle came from the headstock or the bridge... But there was a "rattle" every note I played on that string...
Had something similar- was driving me crazy until I realized what was causing it. The nut that held the tuner post to the headstock was loose and vibrating every time I played the string. It was more apparent playing open fret on that particular string.
So to the op, a few things you can do, check all the screws and nuts on your bass to make sure nothing is loose, make sure the neck is straight, check the saddle for that string- sometimes it's the spring that might be too loose as it has no tension (or lost it's tension) causing it to vibrate. You can also try to lower your pickup height- perhaps it's too sensitive and too close to the strings. If all fails, a tech should perhaps look at it and try to isolate the problem.