Yea that gets your attention!!
You said it still will do it with the amp levels turned all the way down.
Has anyone ever noticed any level metering, clip indicators, ect on any of the equipment
when this happens.
On the board by chance have you noticed any channel peak with an extreme amount
of signal level when you hear the noise, I'm sure you see all the open mic channels pick it
but this would be a slammed peak on a channel.
As for the monitors, is it in the wedges and in the M1 ear monitor system, that could hard for them
to tell if it is actually in the monitor system or they could just be hearing the sound picked up
from the open mics and that being fed into the monitors.
Do you have any direct board out recordings made at the same time you heard noise, if so
what's does the recording sound like, if you don't make a stereo maybe try recording
one till you catch the noise on a recording.
Maybe for the sake of testing set a multi track recording system to hopefully
catch something on one of the input channels.
Do you have phantom power on any channel.
Does it do it with the board muted and or every channel and the mains turned down.
Try moving/wiggling every connector and cable starting on the stage, to the board, to the amps, don't forget the power cords.