Probably some kind of interference. Try changing channels.
Side note you should never have your pack up to the level that would "hurt your ears". People say, I'd never use IEMS because I don't want to blow out my hearing if someone drops a mic or something. limiter or not, if it's not up too loud it's impossible to hurt your ears.
Does it have multiple channels?
where is it in relation to where you are on stage? clear line of site? I say this because I get what you are talking about if it's between me and the drummer for some reason.