Audio problem

We were setting up for an event and have run into a strange white noise problem.

After we set up our mics we were fine but then suddenly started getting white noise through our BlackMagic. It doesn't seem to matter what I unplug (including the audio feed) there is a constant feed of white noise going to the switcher, reading on the monitors, the ATEM software, etc.

Thoughts?

Mains interference from an electric motor (pump or fan) perhaps? Or something radio based?

Is it really all frequencies? You could try the equaliser and if you are lucky you can find something to filter some of it out.

Tim