It is a good idea to read the link that Mark referred to. It could be many different things from RF interference to mechanical issues. Check the nut on the jack on the Sennheiser G3 body pack if they get loose I have heard similar noise. I have had a persons clothing (I think they were wearing silk) cause a noise that was really hard to trace.
Don’t forget proper frequency coordination in the environment. Don’t trust what the manufactures say is a good combination.
I consistently test all of the wireless body pack mics that I use before they are put out for the actors. While wearing headphones I cue up each one and talk into them listening for anything that is not normal. I then hold the pack up in the air with the mic dangling and I run my fingers down the wire. If there is a problem and many times if there is one that is about to develop a problem you will hear it when you do that. While listening I manipulate (wiggle) the antenna and the mic connecter making sure everything is tight on the pack, like the loose nut thing I mentioned. I then talk into the mic again and if all checks out ok it is considered good to go for the show.
Could you be having sweat problems? Also I would never let a person have another wireless device like a phone on them. Have you ever heard what the iPhone handshake issue sounds like? I can’t remember ever having a presenter refuse to let us clip the pack on their belt. This was before ear set mics, we used MKE2 mics on Lectrosonics packs. And we usually real neatly rigged the mics on their clothing.
There was this one presenter who always came wearing a dress and I would hand her the mic and pack and send her off to the bathroom for her to attach it to were it worked for her. And she would come back with the mic sticking out near her neck and I would deal with mic placement. I remember one time after she was done she wanted to get the pack off without having to go back into the bathroom so in the corner of the room I stood with my back to her blocking any ones view and she removed the pack and handed it to me. I remember the pack being very warm. The funniest part was every time she would see me to get wired up it was like she was just remembering she was wearing something that required her to go thru this routine to get mic-ed up.
I once had to mic up one guy (I am pretty sure he is or was one of the riches men in the world) and he was a handful. He was so hyper and fidgety it was a challenge to get him to stand still enough. I just felt like telling that story.
All sounds very familiar! One CEO was insistant he did not want the head mic with a pop shield saying it distracted him another insisted it must be brown (we obliged). Most women CEO get fussy about you touching their hair! (reason for using single ear units)
Actors are probably a lot more disciplined than delegates which helps keep issues to a minimum,your advice is all good another thing I also do is monitor the radio receiver racks with no TX units switched on to ensure no one else is on the frequency in nearby hotels or conference rooms anything spurious flashes up from the red mute dispay.
Phones are definately a suspect so will look to clipping the packs to belts only as many do not like being parted from their handsets!