nice reply Kevin.
and yes, I was going to even suggest that the FOH guy watch his channels close..it could be a phone on one of the players on stage.. I've made the mistake of placing my iphone on top of my bass amp..talk about an antenna !!
also, not sure the original post mentioned how loud the noise is...can you gate for that, or is it as strong as the main audio signal?
A gate wouldn’t solve the problem because the interference it is usually loud enough that it is hear with the original sound. It isn’t buried under it and just needs to be hidden when there is no other sound on that input.
Just yesterday I was at a church consulting about their sound system and the pastor said that he messed up the wireless mic tuning because he heard a problem with his mic. And was trying to find a better frequency. He described the sound and I asked him if he had an iPhone that was on in his pocket and he said he was pretty sure that he did. I sent him the frequency coordination report that I had done last year and he was able to get things back to normal. I suggested that he test his mic with all the other wireless devices on (but not all in a pile) and see if the problem has gone away.