Those little devils can be obvious (loose hardware, bad string) or hard to find (sympathetic vibration in a loose brace).... but whatever the source, here's a great place to start the search. Go to www.frets.com (Frank's sister site), go to the "items for luthiers" section and find the "buzz diagnosis" page.... and the eventual answer will be there.
First just try changing the string. Then check the nut and make sure the slot for the string is cut at the right angle to the peg or not larger on the peg head side then the fingerboard side. Sometimes when the angle is wrong or the string is tight in one part of the slot and loss in another part of the slot you'll get a clanging sound.
I was just kidding! All the advices here are right. Give a look to the saddle too. If it's too low or the angle isn't sharp enough, the string should vibrate badly.
Antionio, I figured you were kidding but I had a vision of someone new starting up a thread about what a great luthier Frank's sister must be to make a site like Frets.com.
My imagination says that Frank's Sister's site would be an Arts and Crafts for the home sort of thing. I imagine that she would have a page dedicated to all the kitchen utensils her brother 'transferred" to the garage to make tools when he was a kid.
My first check in this case is : does it make the noise opened or fretted? If open, the nut is the most probable culprit. If fretted, I would check for a high fret or anything loose.
Ned, do you imagine Frank has ever been a kid? It's like to imagine Hogwarts' head teacher - Albus Dumbledore - as a cheerleader!!! And he have no tools! He does everything he does on guitars just with his pocket knife! I'd like to be Harry Potter, to learn from such a master!
Well, Antonio, I got a laugh out of this when I read it . We do seem to suffer a shortage of the "fairer sex" on these pages and I suppose in the industry as a whole. Too bad.