For string buzzing problems in general , it's more likely the bridge is set a little low ... You can easily adjust your bridge (methods of adjustment depends on the bridge's type , Floyd Rose , Standard Whammy , Fixed, etc.).
Or you may have been in a bad mood , and just hitting the string so hardly that it buzzes ! ;)

Unlikely , there may be something (whammy's springs for instance) that's vibrating at the pitch of the 18th fret on the E strings which is A# note.
I just thought of that because you said the buzz comes from two exactly similar notes ... You can check for that by playing an A# note from anywhere esle on the fretboard and see if it also buzzes.