I've recently changed the strings in my guitar, and it has started to buzz. But only in some G notes: 3rd fret in first string, 8th in second string, 12th in third and 17th in fourth. Any reason why or how to solve it? Thanks My new strings are D'Addario, but before I had the same brand.
Something else may be loose elsewhere on the guitar and that is its resonant frequency. Is it like a rattle? Strike a G and feel around the guitar. If you touch something and the buzz stops you have found the loose part. Try around the head stock, the strap buttons, the knobs, the 1/4 Jack, the little string holders on the head stock, the screws that hold on the pick gaurd, etc.
Once you find the loose bit tighten it and you should be good.
If you follow b3ko's suggestions and still can't discover what is buzzing, it could be a part that you can not see or touch, such as the truss rod inside the neck. Try adjusting it just a tiny bit with a truss rod wrench to eliminate the buzz. If that doesn't help, you might take the guitar to a luthier, who can shoot a small amount of white glue into the truss rod channel to stop the vibration.