Is your Mixer battery or wall powered? Have you tried going on battery for everything? ground loops can come from anywhere and this includes anything hooked up to the camera and a wall outlet such as monitors, mixers, speakers, teleprompters etc.
Make sure everything that is pluged into the camera is on the same circut or try ground lifts (the ugly grey things that you plug onto extention cords to work in older houses). The ground lift is a last resort because it produces a few hazzards. Other things can be happening (I once had to deal with an alarm system in the floor that caused a buzz. We had to go all battery and the camera guy was not happy because his monitor was not battery operated. We would et up the shot then unplug the monitor to shoot.
*** Also dimmers on lights cause buzzes make sure they are on a different circut.