when i play B (12th fret, b string, it only happens on that fret.) it makes a buzz sound like something is rattling or loose, but it comes from the bridge, not the fret.
i've tightened the bridge and stuff.
anyone know what it could be?
well first is first. We have to identify the problem. In this case, it's an actual buzz on the guitar. This means you probably can identify it quite easily. Here is how to do it:

1. Play all the frets. All of them. Not just running though them. Make sure when you play them you are looking out for something that might stand out as odd. Take special care in the B string. That's where the problem seems to be coming from. So now you have found out when your guitar makes noise. Onto:

2. The source of the noise. This is easy. Take all of the places you have found that provoke the noise and play. Then, let the note ring and hold a part of your guitar. If the noise stops and you feel some sort of vibration: Bingo! You have found the source. Rinse, wash, repeat with all of the places you hear a noise holding down every part of your guitar.

3. Sorting out the problem. This is pretty straight forward, once you know the problem, you know how to sort it, and if you don't, take it to your local shop and have em teach you.

Finally, your case, sounds like the ball end of the string isn't slotted in properly. Flip the guitar, take out the back cover and look at all of your strings. There might be one ball that looks different(probably the B string.). From there it's simple, just take out the string and re string it. Seeing that it's only one string, I recommend you buy a set and change them all because: new string + all other strings old = not so good. Just in case that isn't your problem, and just for practice, I recommend you do those three things I pointed out up there first. You might find other things you didn't know about. Do it periodically, especially after gigs, travel, or a while without playing.


Somebody should quote that somewhere.
maybe it's a screw in the saddles, maybe it's the ball of the string, maybe it's a spring, maybe it's just that part of the fret