Try changing strings first if you have old ones. Fretbuzz can be caused also by something that i call "fret hole" in string. Look at the picture:

Of course in my picture it is way too big. Actually it is hard to spot it, but a fraction of an millimetre can make big difference.
Holes like this appear sometimes in old strings, especially when you have "heavy" fretting hand and press strings using a lot of force. Hole like this make string sit lower than usual when fretted and make it rattle on the next fret