Does it just have this problem in this one venue? That may tell you if the problem is some interference your picking up from the location.
Start with unplugging the mics. If the noise is still there you may want to consider replacing cables in your rack and see if that get it. If the problem is not in any of your cables, you now have backup cables, something every KJ should have anyway.
If the problem continues ask Lonman
Mojo