Is the feedback just in your in ears or or across the stage? Is unmuting the bus at the board when this starts or the iems themselves?
If it is just in your iem bus, you might have a loop inside the mixer that happens to only be feeding the bus for your ears.
Simplest thing to do is to mute everything including the other buses, and effect sends/returns except your ears and then unmute things one at a time until you locate the source.
You can also go through all your fader banks and look for channels that are showing a constant signal even when there should be nothing present. Check your channel/bus routings and make sure a channel that is part of your in ears mix isn't somehow feeding itself. The x32 line certainty has the ability to created loops internally with the flexible routing that is available.
It is even possible that somehow you are sending the in-ears bus back to itself through ar channel since the channel input can be a bus, which can be sent back to itself.
Once you located the source you may have dig deeper and see what all that source is feeding, or what is feeding into that source.
In could be a long path back to the bus.
