buzzing? sounds like something touching the fan when it's in the case. make sure the cables and stuff is clear of the fan.
not like it's never happened to me... before I got my Corsair 800D (it's so spacious inside!) my case was a bit crowded with cables and this and that... inevitably came the day after a cleaning with the air compressor I heard a buzzing from within the case- found out a cable was interfering with the CPU fan!
alternative is power supply being over loaded... the power needs of the newer GPUs are very different from say, a 9500GT. does the buzzing by any chance is low-frequency- about the same as the 60 Hz electrical mains buzz? then tehre's some issue with the Power Supply or your computer's connection to the electrical supply. (or it could be that there's something wrong with your electricity.. if you have a voltmeter that can measure the mains voltage and know how to do so properly, you can check and see if it's within spec- between 110 and 127 volts.)
I've heard some things making that 60 Hz buzz but never my computer! the 60 Hz buzz often indicates too much power draw, loose connection/bad connection, or when running a heavy inductive load. sometimes lightbulbs makes this buzz too (but very rarely).