Acoustic woes...

I know this is a bass forum, but this the best source of info I got...

I've got a cheap acoustic guitar, and a couple of notes have got what I thought to be fret buzz, but they're the only notes on the string that do it, if you detune it moves with the new tuning, and heres the clincher - it's the same note on each string! So I'm guessing (tell me if this is possible or not guys) that the frequency of this particular note is rattling something in the sound cavity of the guitar. If so, what can I do about it, if anything?

-Bernard.