All good pointers, above, but missing an important troubleshooting step.
I've used this on everything from an old lute to pipe organs to guitar amps to dulcimers to... well, anyway:
Play the offending note and just poke around the instrument with your finger. Prod everything you can think of. Sooner or later you will hit a spot that stops (or changes) the buzzing sound, and that will be your starting point for identifying the problem.
For a buzz to occur you must have either (1) something moving that should be tight or (2) something that is encroaching into the path of something that is supposed to be moving, i.e. a string. It may not be the string or its mountings at all, it could just be that the loose spot is resonating at that particular frequency. Another trick: drop the suspect string a half tone and see if it still buzzes. If not, the problem may be a resonance away from the string. Again, something loose that should be secure.
Hope this helps
-Glenn