Was that the buzz in the air at the event?
Joking aside, if it wasn't something actually recorded, first determine if it is constant through the entire recording and if it is in both channels. Then make some tests trying to recreate the situation. Was there anything electrical in close proximity with the recorder or preamp such as a phone? Doesn't sound like cell phone interference, but could we some more constant EMI interference.
In the short sample it sounds constant in the left channel. If its only in that channel, try swapping channels at the mic capsules, then at the nbox, then at the recorder, to determine if and when the buzz switches sides along with any one particular component. If/when it does, you'll have isolated where it is coming from. May take some custom interconnects or creative temporary patching to do so.