[mirwa]

Its hard to diagnose things via the internet.

But.

I would first look to see if you had a loose fret, then if frets are good I would check that the fretboard is not worn in the middle area between the frets, if its worn you will be getting a more substantial pull down onto the fret, this may change the strings dynamic and cause buzzing.

Again, hard thing to diagnose via a forum.

Steve