Title's pretty self-explanatory. Don't think the buzzing's coming from the frets. Could it be the nut? What should I do?
if you're sure it's not coming from your frets (so, your neck is straight and your action isn't too low) then you'll have to purposely make the buzz happen and feel around to identify the source. hopefully it's something that's a little loose, and the buzz will stop when you touch it. I had one of my tuners come a little loose that was buzzing when i hit bass notes. it's not likely to be your nut, that should be held firmly in place by the strings even if it's not secured.
your buzzing could be from many places, and like mike_oxbig says, be sure and check your tuners, as i've seen loose tuners that buzzed on several guitars. but it could also be from too dry an environment. are you using a hygrometer that you tested to keep track of the humidity where your guitar is kept?