I get that buzzing...sort of like an electrical sound, on all of my bikes using "HH" rated semi-metallic pads. Never an issue.
If you hate the sound, try some of the Galfer Black rear pads..the cheap ones G1054 series (GG rated). Plenty of power for the rear and a decent feel. They also polish up the rotor and make it look really nice. VERY QUIET! I really dislike Galfer, but these pads are really good rear pads.